Method and apparatus for high-voltage D.C. transmission with a bypass circuit for malfunctions

ABSTRACT

A method and apparatus are described for operating a high voltage d.c. (HVDC) transmission line system connecting two a.c. transmission line systems during normal operation and during malfunction in either the rectifier station or the inverter station of the HVDC system. 
     When a malfunction exists in one station of a HVDC transmission line, such as a rectifier station, then the regular thyristor firings are disabled and a bypass circuit, preferrably one or several bridge paths of a converter, are fired. As soon as a corresponding change in the current or voltage occurs at the d.c. voltage connections of the other station, this station assumes rectifier operation during which the HVDC transmission line is utilized as a reactive load to stabilize the other system. Once the malfunction has ceased to exist, operation is first resumed in the first station by extinguishing the bypass thyristors and subsequently resumed also in the other station. This procedure permits stable normal operation in both stations, or bypass operation without using remote control signals during which the HVDC transmission line can be rapidly controlled to stabilize the systems.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is related to the following co-pending applications:

"Method and Apparatus for Resumption of Normal Operation of a HighVoltage D.C. Transmission Line" by Helmut Neupauer, U.S. Patent OfficeSer. No. 706,754.

"Method and Apparatus to Operate a High Voltage D.C. Transmission SystemWith Automatic Control of the Converters" by Helmut Neupauer, U.S.Patent Office Ser. No. 706,711.

BACKGROUND

This invention relates to a method to operate a high-voltage d.c. (HVDC)transmission line between two a.c. networks during normal andmalfunctioning conditions and an apparatus for said purpose.

In this apparatus a first converter is connected to a first a.c.network, NA, and normally operating as a rectifier, feeds d.c. currentinto the HVDC transmission line at the first station, A, while a secondconverter, mounted at a second station, B, and feeding into a seconda.c. network, NB controls the voltage of the HVDC transmission line. Anymodification of the HVDC transmission line voltage determined by thesecond station effects a change of the current fed into the firststation after a lag time determined by the transmission time or traveltime of the HVDC transmission line, as long as the control angle of thefirst converter is not reset. Conversely, any change of the d.c. currentsupplied from the station, A, effects a change of the commutation timeof the thyristors of the second converter, whose control angle has to bemonitored and, if necessary, shifted accordingly in order to maintainthe inverter step limit.

For that reason, the two stations are generally connected by remotecontrol lines and interact, e.g., by a so-called marginal currentsignal, to form the control angle. The controllers installed in thestations are set for relatively slow action in order to allow time forcontroller interaction and for settling time.

If a voltage short circuit occurs in the station, A, then shortlythereafter the d.c. current in the HVDC transmission line lapses or thesecond converter is shut down in order to avoid further power supplyinto the short circuit a, e.g., rectifier malfunction. The d.c.connections of the first converter can be short-circuited via ashort-circuit switch referred to as a bypass circuit in order to detourany existing d.c. current by the converter and maintain the current inthe line at a reduced voltage.

This short-circuit switch can be designed either as a relativelyslow-acting mechanical switch or as a thyristor appropriately rated forrelatively low currents. When installed, said bypass-thyristor is notsuited to carry current supplied by the station, B, in case said stationhas not been shut down, but instead is switched over from inverteroperation and is operated as a rectifier during the malfunction. Such abypass current would mean a reactive current for the a.c. system, NB,because the HVDC transmission line becomes a long current chargedinductor and could involve high levels, e.g., 60 percent, of the nominalcurrent.

While the converter thyristors available in any event for normaloperation of the first converter could carry such a current, they couldonly be fired using expensive firing devices if due to the rectifiermalfunction the HVDC transmission line had gone dead and if also themalfunctioning system, NA, could not provide firing energy.

For that reason said bypass operation in which converter A is bridgedafter a rectifier malfunction and converter B is operated as a rectifierhas not, heretofore, been considered a reasonable approach simply forcost reasons.

Furthermore, the control mechanisms of the station, B, are set forrelatively slow action for the reasons explained above, and said bypassoperation would only result in a reactive load for the system, NB, whichwould be difficult to control.

In case of a malfunction in the station, B, similar conditions wouldprevail. During every voltage short circuit the HVDC transmission linecurrent would flow into this short circuit, and the inverter would fail.It is shut down, and the HVDC transmission line discharges via theinverter thyristors until the current goes out. A short-circuit switchalso installed at that point can be closed until voltage resumes, andduring this malfunction rectifier A is shut down. If the converterthyristors themselves were used as short-circuit switches, i.e., thebypass circuit, then additional measures for correct selection of thefiring pulse would be required to fire the thyristors in accordance withthe system cycle when voltage resumes.

A further complicating factor is that the remote signal lines havetransmission and processing time factors of their own so that thesignals required to operate the functioning station regarding therespective status of the other station are only available aftersubstantial delays.

For that reason heretofore normal operation of the HVDC transmissionline has been mostly designed in such a manner that, for example, afailure of the inverter or other malfunctions were avoided, if at allpossible; however, when a malfunction occurred, the HVDC transmissionline would be held without current by shutting down both converters.

An object of this invention is thus to define a mode of operation of theHVDC transmission line whereby the HVDC transmission line can beutilized even in the event of a malfunction in one of the stations.

It is another object of this invention to provide a method for operatinga HVDC transmission line between two a.c. networks continuously duringnormal and malfunctioning conditions.

It is a further object of this invention to provide a control methodresponsive enough for operation during malfunctioning conditions.

SUMMARY OF THE INVENTION

Briefly stated in accordance with one aspect of the invention, theforegoing objects are achieved by providing a method for operating anHVDC transmission line system connected between two alternating currentsystems, a first network and a second network, during emergencyoperation resulting from a malfunction having a first station connectedto the first network which during normal operation operates as asystem-synchronous first converter rectifying the incoming alternatingcurrent and impressing a normal d.c. current through the HVDCtransmission line, and a second station, connected to the secondnetwork, determining the normal voltage of the HVDC transmission line,further forming a leading release signal subsequent to the end of themalfunction; wherein the second station during normal operation operatesas a system-synchronous second converter inverting the incoming HVDC andimpressing a normal a.c. current into the second network; said methodbeing adapted to form a malfunction in either situation. This methodcomprises the steps of forming a leading fault indication signal at theonset of the malfunction in the malfunctioning station; forming thebypass circuit from bypass thyristors which are designed to withstand asa normal operating condition a load current corresponding to the levelof reactive current encountered during the malfunction bypass operation,by firing said bypass thyristors by means of said leading faultindication signal; forming subsequently in the functioning station aderived fault indication signal; stimulating a rectifier operation ofa.c. current into bypass d.c. current in the functioning station by saidderived fault indication signal; impressing said bypass d.c. currentthrough the HVDC transmission line and the bypass circuit; interruptingthe bypass circuit causing said bypass thyristors to becomenon-conducting by the leading release signal subsequent to the end ofthe malfunction; initiating a transition of the previouslymalfunctioning station to system-synchronous normal operation by theleading release signal; forming a derived release signal in thepreviously functioning station; terminating said rectifier operation bythe derived release signal; and further initiating from the derivedrelease signal a transition of the previously functioning station fromsaid rectifier operation to system-synchronous normal operation.

In this invention the malfunctioning station is bridged by a specialcircuit to handle the short during a malfunction designed to withholdthe load by a reactive current reflecting the requirements of the a.c.voltage system in the functioning station, and the functioning stationis used as a rectifier to supply the current. The HVDC transmission linecan therefore be used to provide a rapid reactive load, to dampensubsynchronous resonances, or for other balancing procedures,particularly for a maximum stabilization of the functioning a.c. system.This can be achieved, in particular, since in the event of a malfunctionthe control mechanisms of the functioning station do not have to becoordinated with the bridged other station with respect to their controlspeed.

This task is solved by a method defined in accordance with claim 1. Thesecondary claims further develop additional preferable modifications ofthis method, particularly taking into account the various types ofmalfunctions which could arise, as well as detailing the arrangementrequired along with additional preferable modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter which is regarded as theinvention, it is believed that the invention will be better understoodfrom the following description of the preferred embodiment taken inconjunction with the accompanying drawings in which:

FIG. 1 shows the design of a HVDC transmission line system.

FIG. 2 shows the relationship between firing angle α, extinction angleγ, the inductive d.c. voltage drop, the d.c. voltage and the a.c.voltage when using a converter synchronous with the system.

FIG. 3 is the schematic design of a HVDC transmission line shortcoupling with automatic control of the control angle for normalfunctioning operation.

FIG. 4 shows the configuration of the HVDC transmission system inaccordance with FIG. 3.

FIG. 5 shows a modified design of a HVDC transmission system withautomatic control.

FIG. 6 Another schematic design of a station, B, with automatic control.

FIG. 7 is a design modified from FIG. 3.

FIGS. 8 and 9 are modified designs of a station, B, in contrast to FIG.6.

FIG. 10 shows another schematic design of a station, A, with automaticcontrol.

FIG. 11 is a design of a monitoring device for either a station, A, or astation, B,

FIG. 12 or 28 are signal patterns and design of a type of limit valuewarning device for the monitoring unit in accordance with FIG. 11.

FIG. 13 shows the use of a network model in a station with a HVDC remotetransmission line.

FIG. 14 shows the design of the network model of FIG. 13.

FIG. 15 shows the detailed design of a station using a station, B, as anexample.

FIG. 16 are signal patterns for operation of a station, B, in accordancewith FIG. 1 when resuming the system-synchronous operation after ashutdown of the HVDC transmission line.

FIG. 17 shows the circuitry of a clamping circuit for the arrangement asshown in FIG. 1.

FIGS. 18 and 19 show the patterns of currents and voltages as well ascurrent transit times for the converter thyristors when operatingaccording to FIG. 16.

FIG. 20 shows signal patterns for operating a station, B, in accordancewith FIG. 1 in case of a temporary malfunction, i.e., transition fromnormal operation and into normal operation.

FIG. 21 shows the circuitry of a selector switch in the design accordingto FIG. 15.

FIG. 22 shows the pattern of a returning a.c. voltage following a systemmalfunction and the system-synchronous firing pulses of the station asshown in FIG. 15.

FIG. 23 shows the patterns of control angles, voltages and currents in apreferred example.

FIGS. 24 and 25 show the patterns of the voltages, currents and signalswhen operating a HVDC transmission line with a de-energized lengthduring a malfunction of a station, B, or a station, A.

FIGS. 26 and 27 show the same patterns in the event of emergency bypassoperation during a malfunction of a station, A, or a station, B.

DETAILED DESCRIPTION

FIG. 1 shows an a.c. system, NA, connected to a HVDC transmission linevia a station, A, and an a.c. system, NB, connected to the HVDCtransmission line via a station, B. Both stations contain a converter.If using this arrangement d.c. current is to be transmitted from thefirst station, A, to the second station, B, then the current flowingthrough the HVDC transmission line will be preset by having the firstconverter located in the station, A, operating as a rectifier in orderto draw from the first system, NA a current at voltage U_(A). The outputd.c. current i_(dA) of the converter of the station, A, is thenimpressed into the HVDC transmission line. The second converter locatedin the station, B, operates in this mode as the inverter in order tosupply the input d.c. current i_(dB) received via its d.c. connectionsinto the second system, NB, with the inverter control angle used for thecurrent feed controlling the input d.c. voltage U_(dB) of the station,B, and thus the voltage level of the HVDC transmission line.

Generally, efforts are made to operate the converter with low levels ofharmonics, for which reason the converters are designed as 12- andmore-pulse converters containing numerous d.c. current sideseries-connected component converters, for example, connected to theHVDC transmission line using converter reactance coils LA or LB and/orfilter circuits (CFA, LFA and CFB, LFB, respectively) and connected tothe respective a.c. voltage system, NA or NB, using convertertransformers with differing circuits characterized by theirtransformation ratio u where typically u=1. Particularly for shortcouplings serving to link two closely adjoining systems and oftencontaining only a high-voltage smoothing reactor, the use of the filterelements LFA, CFA and LFB, CFB respectively can often be omitted. Thecurrent appearing following the filtering elements is called the "HVDCtransmission line current" and is designated with i_(dLA). Thecorresponding HVDC transmission line voltage is designated U_(dLA),while the quantities before the filter elements are designated by i_(dA)and U_(dA).

The actual values required for control purposes are generally obtainedas close as possible to the HVDC transmission line connection point ofthe respective station, i.e., possibly behind the filter units; in othercases, e.g. to monitor the HVDC transmission line under operatingconditions, it is irrelevant where the (not depicted) measurement unitsrequired to obtain the actual values are installed. The componentcurrent converters 1A', 1A" are series-connected on the d.c. currentside to connect poles 2,3 of the HVDC transmission line and eachcontains one output phase R, S, T of the thyristor groups correspondingto their transformers designated by "+" or "-", if respectively theiranodes or cathodes are connected to the transformer. Thus, for example,thyristor group R⁺¹ is located in the direction of current flow betweenthe transformer and pole 2. A drive unit supplies the component currentconverter 1A' with the firing command sequence S'α which consists of theindividual firing commands R⁺ α' . . . T⁻ α' and is either disabled by aclamping circuit shown by switch symbol BS' or amplified to form afiring pulse sequence Zα' as individual impulses R⁺ ' . . . T⁻ 'connected to the thyristors designated with the same symbols.

The drive unit ST'_(A) contains a reference voltage system U'_(Asyn)from a reference voltage generator RG'_(A) connected to the a.c. voltageinput of the component current converter, which forms the firingcommands Sα' by comparing U'_(Asyn) with a control quantity, e.g., acontrol voltage U_(STA) or a control angle α_(A). System-synchronousoperation is achieved when control angle equals α_(A).

The control quantity, for example, control angle α_(A), is supplied by acontrol device 4A and according to the state-of-the-art is generallyshared by all component frequency converters of the station. Thecomponent current converter 1A" with thyristor groups R⁺ " . . . T⁻ "and its control devices RGA", STA", BS" is designed in the same way asthe component current converter 1A'; similar quantities are designatedaccordingly. As in most cases it is obvious to those skilled in the arthow to control the existing component frequency converters by using thecontrol quantity of the station, A, the overall current converter isoften designated as 1A in the following description and thedifferentiation of the quantities assigned to the respective componentcurrent converters suppressed.

The Station, B, is designed analogously as far as possible; thecomponent frequency converters 1B' and 1B", for example, are oftentreated as one single converter 1B. As many structural components anddesign features are identical for both stations, the differentiationusing the letters A and B is omitted in these cases.

In the converter 1A operating as a rectifier, the control angle ispreset near the wide-open setting (α_(A) ≐0) and initially controls theoutput d.c. voltage U_(dA). The output current i_(dA) is then determinedby the voltage drop U_(dLA) -U_(dA) at the filter choke coil LAaccording to the following equation:

    i.sub.dA =1/LA∫(U.sub.dA -U.sub.dLA)dt                (1)

Thus, if a control quantity i*_(dA) is fed to the control and regulationunit 4A as a set value for a current control supplying the control angleα_(A), then the collapse of voltage U_(B) or U_(dB) as, for example,brought about by an inverter failure in the station, B, or any otherchange in the operation of the station, B, after the HVDC transmissionline travel time, results in a change in U_(dLA) resulting in a currentchange and excitation of the current controller.

For current control of the station, A, thus U_(dLA) or with the lag timedetermined by the HVDC transmission line travel time U_(dB) operates asthe fault indicating quantity.

The situation is similar if the active power to be transmitted is usedas the control quantity of the station, A. In this case an active powercontroller supplies, for example, the set value i*_(dA) in accordancewith the active power nominal value P*_(A) coodinated with the energybalance of the system, NA.

Also, the d.c. converter 4B of the station, B, determines by its controlangle α_(B) the output d.c. voltage U_(dB). As the d.c. current i_(dB)supplied as active and reactive current into the system, NB, isimpressed by the station, A, the control and regulation unit 4B cancontrol the reactive output into the system, NB, in accordance with areactive power set value Q*_(B) which can be used as the controlquantity to stablize the system voltage. Thereby the station B,determines which voltage level will be established in the HVDCtransmission line. The current i_(dB) and the current flow in thesystem, NB, develop freely; it corresponds, except for slight resistanceof losses to the conductor, to the impressed current i_(dA) definedprior to the line travel time.

As particularly for a high active current component of the transmissiona control angle normally near the inverter wide-open control setting(α_(B) near 180 degrees) is the goal, the time lapse during commutationfrom the firing of the succeeding thyristor (firing angle α_(B)) untilcomplete deactivation of the preceding thyristor (i.e., up to theextinction angle, γ_(B)) is relatively long and increases as the currenti_(dB) rises. During the commutation time the voltage U_(dB) collapsesby a so-called inductive d.c. voltage drop.

The inductive d.c. voltage drop brought about by the d.c. currenti_(dB), or the impressed current i_(dA), thus functions as the faultindicating quantity for converter d.c. converter 4B.

This is particularly important since the extinction angle must notexceed a maximum value, the inverter step limit, which depends upon therelease time of the converter thyristors so that no inverter failurewith short-circuiting of voltage U_(dB) arises. An increase of theimpressed current i_(dA) produces. after the HVDC transmission linetravel time, an increase of the commutation time in the station, B andan increase of the inductive d.c. voltage drop which has to be dealtwith by an advance of the firing time point of station, B, i.e.,reduction of α_(B), if the maximum extinction angle or a presetextinction angle γ* used as the control variable of the station, B, isto be adhered to.

Due to these mutual control fault indicating quantities the controls ofthe converters coupled to each other using the HVDC transmission linehave to be coordinated in their operation. Generally, informationregarding the operating status of one converter, e.g., a marginalcurrent signal or a malfunction signal derived from the respectivecontrol quantity) is transmitted to the other station using remotecontrol lines. Due to the travel time of the HVDC transmission line aswell as the processing time for this information transmission, stableoperation of the HVDC transmission line can only be attained if thecontrollers of both stations are set relatively slowly, e.g., on/offcontrol times of 200 ms.

The HVDC transmission line can thus stabilize the relevant a.c. voltagesystems given rapid malfunctions to a limited extent only. Moreover,rapid startup of the HVDC transmission line, in particular given atransitory malfunction in a station and thus a temporary failure of theHVDC transmission line for example, is not possible. Thus, for example,provision of an adequate safety margin from the inverter step limit isintended primarily to avoid an inverter failure at the expense of areduction in the active power transmission.

The operating procedure described below reduces these problems. Itpermits, given adequate protection against inverter failure, highlydynamic control and rapid startup after malfunctions.

Thus, initially the formation and transmission of the informationspecified is simplified and by suitable measures, namely automaticcontrol of the control angle of one station using the fault indicatingquantity or else a model value to be considered, the starting controltimes are substantially shortened. In the case of a HVDC transmissionline short coupling in which, due to the close proximity of bothstations, the information regarding the operating status of the otherstation is available without extended transmission times; a d.c. voltagedetector for the HVDC transmission line voltage is no longer required.

In a HVDC line long-distance transmission the information required inone station regarding the other station is practically formed by theoperating quantities only, particularly actual and set values, of theone station. Thereby remote control lines are omitted and theinformation is available at the earliest possible point in time, namelyas soon as the status change of the other station is noticed in theformer station. The automatic control apparatus thereby initiates veryrapid closed loop control circuits, with startup times of less than 50ms, e.g., 20 ms, becoming feasible.

If the operating uncertainty, due to the model fault indicator quantityused, occassionally leads to inverter failure or to another convertermalfunction, the economic consequences of such a malfunction can be heldto a minimum since the automatic control utilized permits a rapidrestart of the HVDC transmission line following a malfunction. Thus theoperating procedure can be coordinated primarily for optimum utilizationof the HVDC transmission line. In particular, the HVDC transmission linecan also be designed to stabilize dynamic processes, e.g., balancingprocesses in the systems, primarily in order to stabilize the systemvoltage. Particular design features further permit this utilization ofthe HVDC transmission line even when due to a malfunction of a rectifieror inverter the active current transmission is interrupted.

THE INDUCTIVE D.C. VOLTAGE DROP AS THE FAULT INDICATION QUANTITY

The method in accordance with this invention is based upon aninvestigation of the effect that d.c. quantities of a converter have onits operation.

FIG. 2 depicts in broken line the voltage patterns of the individualphases of an a.c. system (system voltage U≈. Assuming that the currentin the converter is commuted immediately and completely when thethyristor is fired, the voltage u_(di)α (t) dependent upon the firingangle α arises at the d.c. connections which is designated as theinstantaneous value of the ideal unsmoothed no-load direct voltage.

The ideal-type assumption that the current is immediately commuted, sothat the commutation time span is zero, is present only under no-loadconditions in which the d.c. current itself becomes zero. Givenwide-open control setting, the average value of the ideal no-load directvoltage under these ideal conditions is the voltage U_(di) which isproportional to the momentary system voltage amplitude U≈ or theeffective system voltage in accordance with a definable ratio which is apure numerical value for the respective converter type:

    U.sub.di =const·U≈                        (2)

i.e.,

    U.sub.diA =const·U.sub.A, U.sub.diB =-const·U.sub.B

The change in the value for the station, B, takes into account thereversed current orientation of the thyristors of converter 1B incontrast to the numerical orientation of voltage U_(dB). U_(di) respondsto the voltage vs time waveform of the rectified system voltage.

The respective ideal unsmoothed no-load direct voltage u_(di)α (t) andu_(di)δ (t) correlate to the two firing angles α and δ. Their integratedvoltage vs time waveforms shown crosshatched are given as smoothed idealno-lead direct voltages by:

    U.sub.diα =U.sub.di cos α, U.sub.diδ =U.sub.di cos δ

Turning now to actual conditions, it is assumed that the converter withfiring angle α is fired and that the current i_(d) actually flowing viathe thyristors in the d.c. voltage supply requires a certain commutationtime i e.g., corresponding to an overlapping angle u during which boththyristors are current-conducting, in order to pass out of thepreviously fired thyristor. The end of the commutation time is specifiedby the angle δ, i.e., the extinction angle γ=180°-δ so that thefollowing applies:

    180°-γ=α+u                              (4)

whereby the positioning of the extinction angle γ, i.e., the value ofthe overlapping angle u, depends upon the magnitude of the commutatingcurrent i_(d). The unsmoothed d.c. voltage u_(d)α (t) actually arisingat firing angle α is also depicted in FIG. 2. It is practically theaverage value 1/2. (u_(di)α (t)+u_(di)δ (t)) which thevoltage-commutation vs. time slot depicted crosshatched on the rightdivides in half. Thus, for the actual average value U_(d)α of the d.c.voltage

    U.sub.dα =1/2(U.sub.diα +U.sub.diδ)=U.sub.diα -1/2(U.sub.diα -                                    (5)

i.e., the actual d.c. voltage deviates from the ideal no-load directvoltage U_(di)α =U_(di) cos α corresponding to the firing angle α by avoltage differential. This so-called inductive d.c. voltage drop isproportional to the actual d.c. current i_(d) via a converter-specificparameter dx:

    1/2(U.sub.diα -U.sub.diδ)=dx·i.sub.d =1/2U.sub.di (cos α+cos γ)                                      (6)

so that the following relationships prevail: U_(d)α =const·U≈ cosα-dx·i_(d), i.e., given suitable standardized measurement values U_(A)and U_(B) for the a.c. voltage amplitudes in both stations, thefollowing equations apply for the HVDC transmission voltages U_(dA) andU_(dB) :

    U.sub.dA =U.sub.A ·cos α.sub.A -d.sub.XA ·i.sub.dA

    U.sub.dB =-U.sub.B ·cos α.sub.B -d.sub.XB ·i.sub.dB

It should be noted that d_(XB) is a negative quantity due to the definedpolarity of i_(dB) and U_(dB).

As due to the existing inductances the d.c. current practically does notchange during a commutation, equations 4 and 6 permit a precalculationfor each firing time point based on the firing angle and the measurementvalues for the a.c. voltage and the d.c. current with regard to thevalues that the overlap angle, the extinction angle and the d.c. voltagewill assume during firing.

NORMAL OPERATION WITH AUTOMATIC CONTROL

The first application of these relationships is depicted in FIG. 3 for ashort-coupling arrangement, i.e., the HVDC transmission line consists inthis case only of one inductance L=LA+LB, arranged without filteringcircuits between the two converters 1 and 2. FIG. 4 depicts thestructure of this arrangement, whereby inductance L of the HVDCtransmission line analogously to equation 1 is depicted by an integratorhaving integration constant L and with the input value U_(dA) -U_(dB)and the output value i_(d) ≐i_(dA) ≐i_(dB).

The action of conveter 1A of drive unit STA and of the reference voltagegenerator RGA depicted as the current-impressing assembly SRA isdetermined by the fact that the set firing angle α_(A) given withincomplete synchronization of the reference voltage generator to theactual phase of the a.c. voltage of NA determines the voltage U_(di)αA/U_(A) =cos (α_(A) +δα_(A)) except for an angle error δα_(A) (cosgenerator 410 in FIG. 4). By renormalizing (multiplier 411) and takinginto account the inductive voltage drop d_(XA) ·i_(d) of this converter(proportional element 413, subtraction element 412), one eventuallyobtains U_(dA) =U_(di)α -i_(d) d_(XA).

The lag time of the current impressing is symbolized by lag time circuit414, while a dynamic circuit 415 shows the smoothing of the controlangle in the drive unit or in a generally advantageous integral driveunit smoothing mechanism.

This demonstrates that the inductive voltage drop of converter 1A can becompensated in accordance with the principle of automatic control by theaddition of a corresponding model control voltage d_(XA) ·i_(d) to thecontrol quantity for U_(A) (in this case the signal U_(A) ' cos α_(A) *supplied by a current regulator 41A). In many cases, e.g., if a U_(A)normalized arcos-function network 40a is connected following thecontroller 41A to linearize the control characteristic function, thenormalization of the automatic control voltage, model fault indicationquantity, corresponding to the normalization of the control quantity isnot even necessary so that computing circuit 43A only requires aproportional circuit 431' in order to form the model fault indicatorquantity i_(dA) ×·d_(XA) from the available current-measurement valuei_(dA) (in this case i_(dA) ≐i_(d) ≐i_(dB), i.e., a quantity availableat the location of the station, A).

If one connects in addition or as an alternative to the automaticcontrol device (additional element 42A) the voltage U_(dB), then alsothe effect from the converter 1B on the current control will becompensated. The station, A, thus compensates, practically without anydelay, jumps in the fault indication quantity, coming from the station,B, and the controller can be more rapidly adjusted since thereby thedelaying effect of the HVDC transmission inductivity is obviated.Correspondingly, the controller can be optimized strictly in terms ofthe time behavior of the current impression (actual value and set valuegeneration as well as smoothing and delay of the converter) withoutconsidering the time constants to be allocated to the transmissionconductor and the operation of the station, B.

The HVDC current transducer basically required for this operation can,however, be omitted in accordance with the invention if the actual valueU_(dB) for the automatic control is replaced by a model fault indicationquantity U_(dB) computed by a computing circuit 44 from the measuredvalue i_(d), i.e., the HDVC transmission current or the input d.c.current of the converter 1B in accordance with equation (7):

    U.sub.dB =-U.sub.B ·cos α.sub.B -d.sub.XB ·i.sub.dB

from the actual value i_(dB), the system voltage U_(B), and the firingangle, control angle αB. In accordance with the circuitry symbols (FIG.3) a proportional link 442 and multiplier 441 and a summation point 443,for example, serve that purpose.

In this context, however, the time behavior of both converters withtheir drive units and the HVDC transmission line itself must be takeninto account. This is handled by a dynamic element 440 which ispreferrably designed as several smoothing links connected in series andtherefore permits, in particular, adequate consideration to be given tothe resulting lag times.

In summary, the converter 1A is operated with a control angle α_(A)forming part of a control voltage U_(STA) =U_(A) ·cos α_(A) *+U_(dAv),automatic control voltage U_(dA) can take into account the inductived.c. voltage drop ΔX_(Av) =i_(dA) ·d_(XA), but particularly contains thefollowing fault indication voltage U_(dLA) (for short-circuit coupling:U_(dB) or else U_(dB)) as shown in equation (1).

In the station, B, one must also take into account that the inductived.c. voltage drop also functions as a fault indication quantity on thevoltage U_(d)α determined by the control angle α_(B), in addition to thefault indication voltage caused by a synchronization error δα_(B). Thisfault indication variable has to be eliminated by the control quantitycontroller.

The station, B, shown in FIG. 4 contains the same structural componentsfor block SRB with a drive unit smoothing component 415B (if necessarydesigned as an integral smoothing circuit) and a automatic controldevice comprising a summing point 42B and, depending upon the nature ofthe Δ_(XB) signal and the Δ_(XBV) automatic control signal, alinearizing circuit 40B.

As FIG. 8 shows, initially a signal which determines a set value of theHVDC transmission voltage U_(dB) *=-U_(B) ·cos α_(B) * using a presetangle α_(B) *, can be taken from a control device which computes thisvoltage set value from a control quantity or other operating values ofthe station, B, (controlled operation). In particular, however, α_(B) *(or cos α_(B) * if a linearizing circuit 40B is also connected) can beobtained as the output signal of a control quantity controller.

The corresponding model fault indication quantity is then connected tothe automatic control device (summing point 59, FIG. 8) as an inductived.c. voltage drop d_(XB) ·i_(dB) (or as the corresponding automaticcontrol angle) and onto this preset angle in such a way that the controlangle α_(B) is determined by α_(B) =α_(B) *+α_(Bv), with cos α_(B)=U*_(dB) +d_(XB) ·i_(dB) /(-U_(B)). In the circuit component SRB, thecontrol command for the inverter thyristors is generated by a referenceangle Φ(t) of the reference voltage generated by the reference voltagegenerator RG_(B) which is defined by the condition

    (U*.sub.dB +d.sub.XB ·i.sub.dB)/(-U.sub.B)=cos (α*.sub.B +α.sub.Bv)=cos Φ(t)

or

    α*.sub.B +α.sub.Bv -Φ(t)=0

Thereby a jump in the fault indication quantity can be largely andvirtually immediately compensated even if the control quantitycontroller 41A generating the fault quantity is adjusted very rapidly,e.g. time constant less than 50 ms, particularly approximately 10 ms orless. Here as well the controller can be optimized strictly in terms ofthe time response of its own station without taking into account thetime constants to be attributed to the HVDC transmission line and theother station.

Although this does permit stable stationary operation with rapid controlfunctions, it does not assure the maintenance of a corresponding minimalextinction angle or safety angle corresponding to the inverter steplimit.

However, to avoid inverter failure, the commutation always has to becompleted at a certain, i.e., minimum extinction angle γ*. Therefore thethyristors must always be fired at a phase position Φ(t) of the inverterdefined by:

    Φ(t)=180°-γ*-u

In the ideal instance in which the thyristor firing takes place withoutswitching delay and with ideal synchronization at the phase positionΦ(t)=α_(B), with the voltage drop being exactly obtained by the modelvalue d_(XB) ·i_(dB) /(U_(B)), the control angle α_(B) then must belimited to (α_(B))_(max) =180°γ*-u=arcos (-cos γ*-(2d_(XB)·i_(dB))/U_(B)).

In operating conditions in which said firing angle limitation prevails,the firing condition Φ(t)=180°-γ*-u, 180°-γ*-u-Φ(t)=arcos (-cosγ*-(2d_(XB) i_(dB))/U_(B) -Φ(t)=0 or

    -cos γ*-(2d.sub.XB ·i.sub.dB)/(U.sub.B)-cos Φ(t)=0

accordingly applies in the drive unit with γ* or else cos γ* being avalue for the extinction angle which is preset. This presetting of themaximum control angle α_(Bmax) can be handled by a limiting device 58(FIG. 8) at the input channel for the control angle α_(B) of the driveunit, with α_(B) being preset by a control function and, as was justexplained, capable of being set by automatic control. The limitationthereby also functions as an automatic control with an automatic controlangle α_(Bv) =(α_(A))_(max) computed from γ*.

If equation 7 applies strictly and a set angle γ* is calculated andlimited to γ_(min) from the voltage set value U*_(dB) by U_(B) cosγ*=-U*_(dB) +d_(XB) ·i_(dB), then the angle γ* formed in computing stage43B' (FIG. 8) equals α_(Bv) =arcos (-cos γ*-2d_(XB) ·i_(dB) /U_(B)) or,if no linearization is performed, the corresponding voltage U_(dBv)=-U_(B) ·cos γ*-2d_(XB) ·i_(dB) provides a limit value for the angleα*_(B) or a corresponding control voltage which can be preset in anydesired fashion e.g., by a reactive current controller or anothercontrol quantity controller.

Said control quantity controller can then simply cause the control angleα_(B) and thus the voltage U_(dB) in contrast to the automatic controlangle α_(Bv) to be retracted while with given U_(B) and i_(dB) thevalues U*_(dB) and γ_(min) will not be exceeded. If the angle γ* can becalculated from the control quantity set value as takes place incomputing element 47 of FIGS. 6 or 7, then the angle α_(Bv) =180°-γ*-ucan be directly connected to the drive unit in a controlled arrangementwithout any further adjustment.

In the drive unit the following always applies:

    α.sub.Bmax =180°-γ*-u

wherein u is the computed overlap angle from γ* and the inductive d.c.voltage drop (model fault indication quantity).

The model value 2d_(XB) ·i_(d) /(U_(B)) can thereby be determined frommomentary values for i_(d) and U_(B) for each phase position of thesystem. This means that for each momentary phase position of the systemthe respective inductive voltage drop and thus the overlap angle will beprecalculated. By monitoring the specified firing conditions, therefore,a thyristor firing will be initiated if in accordance with theprecalculated overlap angle the still remaining time period until theset time point of a thyristor extinction (predetermined by γ*) sufficesto complete the commutation.

In actual converter operation a control angle controlling signal α_(Bv)calculated from a set extinction angle γ* in accordance with180°-γ*=α_(Bv) +u causes at the reference angle Φ(t) a thyristor firingwhich, however, takes place at the actual phase position Φ_(o)(t)=Φ(t)+δΦ.sub.α of the a.c. voltage system and, due to a possibleimprecision δu of the computed overlap angle leads to the actual value180°-γ=Φ(t)+δΦ.sub.α. Thus, there is a set/actual value differentialΔγ=α_(Bv) -Φ(t)-δuδΦ.sub.α.

For that reason the differential Δγ is preferrably eliminated using anextinction angle controller 41B shown in FIG. 8 to whose output signalΔX_(B) the precalculated automatic control angle αBv is added so thatfor the drive unit we have the following condition:

    ΔX.sub.B +α.sub.Bv =Φ(t)

with α_(Bv) =arcos (-cos γ*-2d_(XB) ·i_(dB) /(U_(B)).

A controller can, however, be used for another control quantity wherebythe set extinction angle γ* to calculate α_(Bv) will be determined fromthe control quantity set value (compare item 47 in FIGS. 5 or 6) and thecontrol output signal ΔX_(B) will correct the extinction angle tomaintain the control quantity set value.

Here as well, the overlapping angle or the inductive d.c. voltage dropbelonging to γ* will be precalculated from momentary values for i_(dB)and U_(B) at each phase position Φ(t).

Thereby one attains a controlled presetting of the extinction anglewhich is used in FIG. 8 to limit the either controlled or regulatedpreset angle α*_(B). In the preferred design example of FIGS. 3 and 4the controlled preset automatic control angle α_(Bv) is connecteddirectly to the control quantity controller 41B output as the modelfault indication quantity ΔX_(Bv) whose output quantity ΔX_(B) serves tocontrol the control quantity preset extinction angle in terms of cosγ*-cos γ=0. The automatic control device 42B, 40B is thus in this caseconnected to the automatic control voltage ΔX_(Bv) =-cos γ*-2i_(dB)-d_(XB) /U_(B) to generate α_(B) =180°-γ*-u+ΔX_(B). This automaticcontrol voltage serves thus to take into account the inductive d.c.voltage drop and is computed by the 43B computation circuit which can behandled, for example, by the computation components with the computationcircuits shown in the circuitry symbols.

Said taking into account of the relationships here indicated can, ofcourse, also be handled in accordance with deduced relationships inmodified computation circuits which those skilled in the art can developas required.

Thus, in the case of a short-circuit coupling one develops anarrangement coordinated for rapid elimination of operating statuschanges of both converters which requires only simple mechanisms;paticularly the high-voltage component only requires a currenttransducer as a detecting unit.

In FIG. 5 these relationships are applied to the system in which theconverters are connected via converter reactance coils LA and LB andfilter circuits LFA, CFA as well as LFB, CFB to the HVDC transmissionline (short-circuit coupling or remote transmission line).

This arrangement dispenses with any compensation of the slight controlangle shift brought about by the inductive d.c. voltage drop of theconverter 1A. Furthermore, to take into account the fault proceedingfrom the station, B, the HVDC transmission input voltage U_(dLA) is usedin place of the model fault indication quantity U_(dB) as the automaticcontrol voltage U_(dAv) in the station, A.

In normal operation the station, A, requires no remote action signalswhich influence its operation similarly to information transmitted overremote control lines regarding the status of the station, B. Rather, inthe station, A, all the required actual or set value quantities requiredfor normal operation are available. The same applies to the station, B.The automatic control of both stations thereby takes into account themutual coupling of both stations in such a manner that stable operationwith short control cycles is attained.

In the design according to FIG. 5, the linearizing circuit 40B isintegrated into the computing circuit 43B since an extinction anglecontroller is used as the control quantity controller 41B, whose outputsignal ΔX_(B) already defines an angle. The cos γ* required in thecomputing circuit 43B is then formed using a function generator 40B'.

The design also foresees that given sudden irregular changes in therespective control quantities in both stations, the respective changerate of the control quantity set values can be limited by run-upfunction generator 45A or 45B. The other components already familiarfrom earlier figures are identified with the old reference symbols.

In the station, B, optionally either the extinction angle γ, thereactive power Q, or another suitable control quantity can be used tomaintain constant voltage, to dampen balancing processes or for otherdynamic controls. This is indicated by the selector switch 46, wherebythe input of the automatic control device 42B can be switched betweenthe extinction angle controller 41B and other control quantitycontrollers 41B', 41B". For this switchover the design foresees that anauxiliary computing circuit 47 will supply the relevant extinction angleset value γ* or else the cos (γ*) obtainable from the function generator40B based upon the control quantity set values utilized in theparticular case.

The converter parameters d_(XA) or d_(XB), respectively, required togenerate the model fault indication quantity which corresponds to theinductive voltage drop of the converter, can be automatically reset inthe relevant computing circuits since always during each thyristorextinction the atual inductive voltage drop is determined from themeasured firing and extinction angle and compared with the computedvalue in accordance with the following relationship:

    U.sub.B (cos α.sub.B -cos γ)=2d.sub.XB ·i.sub.dB.

In accordance with this comparison the parameter 2d_(XB) can then beadjusted as indicated by the parameter feedback device 48 in FIG. 6.

In addition, FIG. 6 symbolically shows that the drive unit STB generatesthe control signals S.sub.α by monitoring the above-specified firingcondition.

The precalculation of the overlap angle which at the momentary phaseposition Φ(t) of the system always belongs to the set extinction angleγ* as well as the momentary values of i_(dB) and U_(B) can, for example,proceed within a millisecond-frequency in a microcomputer 43B as shownthe case of a short coupling (inductivity L) in FIG. 7. Here as well,use is made of the model fault indication quantity U_(dB) as theautomatic control voltage U_(dAv) of the station, A, whereby thecomputing circuit 44 can be further substantially simplified if onestarts not from the control angle α_(B) itself, but from the presetextinction angle γ* or related quantities generated in the microcomputeritself. In particular, for example, in place of U_(dB), the quantityU_(B) cos γ*-d_(XB) ·i_(dB) can be connected by a dynamic circuit whichimitates the transmission behavior of converters and the HVDCtransmission line to the automatic control device 42A. In addition, FIG.7 shows that preferrably the effective output P of the HVDCtransmissions line is used as the control quantity of the station, A.From the control differential p*-p a superimposed active powercontroller 51 generates the set value i_(d) * of the current regulator41A. This set value i*_(d) can also preferrably be regulated by a faultindication quantity i_(d) at the addition circuit 52 whereby i_(d)=p*/U_(dB) is supplied by the divider 53.

FIG. 8 depicts a design for the station, B, in which the run-up functiongenerator 66B initially supplies a control quantity which (if necessaryvia a drive unit smooting circuit 415B) presets the control angle α_(B)of the converter. The quantity α*_(B) can be preset from a non-depictedcontrol quantity controller, e.g., an active power controller or acontroller for the HVDC transmission voltage U_(dB) or can be preset inaccordance with the already discussed controlled operation bycomputational means. An automatic control device 59 is shown whichcompensates the inductive d.c. voltage drop influencing on the set angleα*_(B) which is supplied by a computing unit 43B". The microcomputer43B" previously shown in FIG. 7 also determines the cosine of anautomatic control angle α_(Bv) which takes into account both the voltagedifference brought about by the inductive d.c. voltage drop between theideal no-load direct voltage relative to γ* and the voltage U_(B) aswell as the angle shift brought about by the inductive d.c. voltage dropbetween the angle α_(B) and the ideal firing angle belonging to voltageU_(B).

The linearization circuit 40B' provides a maximum extinction angleα_(max) for a limiting circuit 58 calculated from 180°-(γ*-U+ΔX_(B)),whereby ΔX_(B) is prepared at the output signal of an extinction anglecontroller 67B.

As long as the limiting circuit 58 is not in operation, the converter isregulated by the preselected value of α*_(B). If, however, thecompensated angle α*_(B) attains the preset limit angle α_(max), theextinction angle control intervenes and the converter is operated withthe controlled extinction angle. If the extinction angle controller 41Bis deactivated, for example, by a short-circuit switch 47B, then onlythe automatic control angle α_(Bv) determines the limit angle α_(max).By the action of the angle limitation 58, the station, A, is thenoperated with a regulated extinction angle.

As a rule, the incorporation of the inductive d.c. voltage dropdescribed assures that the inverter step limit is observed as long asi_(dB) and U_(B) do not change all too drastically during a commutationprocess so that the precomputed transmission angle u coincidesadequately with the actual overlap angle. This permits operation withmaximum firing angle (maximum active power transmission or minimumreactive power). A greater safety margin to the inverter step limit isthus not required as long as it is assured that even in an extreme casein which sudden changes of i_(dB) or U_(B) can lead to a failure of theinverter, the converter switches over to malfunction operation and afterthe termination of a malfunction can rapidly resume normal operation.

In the preferred design of the control and regulating device 4B in FIG.9, a monitoring and programming switch 62B is included which contains amemory circuit 63B (for example, a programmed microprocessor) and amonitoring device 64B which will be described later.

Device 62B activates two selector switches 60B and 61B whose position asshown in FIG. 9 corresponds to normal operation. In this normaloperation the automatic control voltage (automatic control angle α_(Bv))generated by the microcomputer 43B' is connected via switch position P1to the automatic control device 42B and added to the control quantityΔX_(B) of firing angle α_(B) which is generated by a control quantitycontroller, particularly an extinction angle controller 41B. A controldevice 68B not shown in detail can, by repositioning switch 60B, beconnected to the automatic control device so that, for example inemergency operation, the control angle will be generated in accordancewith the requirements of the system, NB, (for example, in accordancewith a reactive output set value Q*_(B)) required to approximatelymaintain a constant voltage.

In switch position P2 another model value is supplied to the input ofthe automatic control device 42B in place of the model value for themodel fault indication quantity α_(Bv). Said model value is supplied bya run-up function generator 66B. The end value of this run-up functiongenerator is determined by values preset in terms of the normal powertransmission of the HVDC transmission line in trouble-free operation.For example, in accordance with a normal value U*_(B) for the HVDCtransmission line voltage computed by a divider 67B as the quotient ofthe nominal active power P* and the nominal HVDC transmission linecurrent i*_(dB) for normal operation.

Switch position P3 is reserved for a case in which the control angleα_(B) has to be temporarily shifted due to the surging hookup of anadditional set value (i.e., to shut down the HVDC transmission line orto pass over into normal operation).

The Station, A, is very similar in design with identical componentshaving the same coding numbers along with the letter A as shown in FIG.10.

Aside from a possibly somewhat different operation of the linearizingcircuit 40A, the station, A, is designed so that the selective switch60A switches the current set value i*_(dA) projected for normaloperation for the current regulator 41A to a current set value which issupplied by a superimposed control device 68A in accordance with therequirements of the system, NA, in case of a malfunction. Moreover, inposition P1 of selective switch 61A the voltage U_(dLA), i.e., the d.c.voltage obtained when using filtering circuits between the filtercircuits and the HVDC transmission line connection of the station, A, isconnected to the automatic control device 42A as the model faultindication quantity U_(dAv) in normal operation.

In both stations, moreover, the startup generators 66A, 66B as well asthe control variable controllers 41A and 41B can be deactivated by thecontrol and programming circuits. In addition, the design foresees thatin the event of a malfunction the devices 62A and 62B transmit relevantmalfunction signals to shut down the converter, e.g., by triggering theclamping circuits BS from FIG. 1.

MONITORING MALFUNCTIONS

In the normal operation described there are no complications due todelay times and processing times of remote control signals which oftennecessitate a slow adjustment of the HVDC transmission line operation.

Normal operation of the stations is, however, possibly only if bothsystem, NA, and system, NB and both converters are intact. Should onesystem or converter fail, the operation has to shift into emergencymode. For that reason both stations require a monitoring device whichmonitors the correct operation in both stations.

Said monitoring device is depicted in FIG. 11 and will initially beexplained in its operation in converter A. Said monitoring and thealternating transition between normal operation and emergency operationgiven malfunctions should also, as far as possible, be operative withoutremote control signals. Rather, based upon a further fundamental conceptof this invention, the required information regarding interrupted orresumed normal operation in the other station should be recognized fromthe effects of said operation in the former station. It would thereforebe advantageous to generate clearly recognizable signals that thetransition in the other station proceed in a certain fashion. Whilethese measures for the transition from and into normal operation will bedescribed later, first the design of the monitoring arrangement based onFIGS. 11 through 14 will be explained.

In these figures the design for the monitoring arrangement of onestation only is shown. The other station contains an identicalarrangement and the signals F_(de) generated in such an arrangement are,if necessary, to be supplemented by the identifying letter of thestation in which they are generated (e.g., F_(deA), F_(dfA), in thestation, A.

The advantage of this monitoring device is that it does not requireremote control signal lines. The start or the end of a malfunction inthe other station is recognized on the d.c. current side input of thefirst station at the earliest possible moment. If for safety reasonssignal lines connect the two stations, then the signals transmittedthereby do not normally directly initiate measures to shut down or startup the first station. Rather, this mode of operation is in the lastanalysis initiated by the HVDC transmission line currents, e.g., instation, A, by measured values i_(dA) and/or i_(dLA) ; and the HVDCtransmission line voltages, e.g., U_(dA) or U_(dLA) ; and the logicalcombination of the measurement variables) particularly the logicalcombination of the measured values for amplitude and, if necessary,phase location of the first station's a.c. voltage system.

The monitoring device is moreover also in the position to notify thesound or malfunctioning status of its own station and to store therelevant notification.

The system voltage amplitude U.sub.≈ (in case of the station, A, thismeans voltage U_(A)) is monitored using a limit value warning device 711to determine whether the preset minimum value required for properoperation has been exceeded; the output signal G₇₁₁ ="1" shows that thea.c. voltage system is intact.

The monitoring signal G₇₁₁ can be used, in particular, to set the outputsignal Q of a malfunction memory 700 to the Q=1 signal corresponding toproper normal operation. The memory is designed, for example, as aflipflop whose setting input is set by an OR gate 701 using a releasingsignal F_(f) which releases normal operation. Said release signal isinitially generated when ending the malfunction of its own system byhaving the resumption of the limit value U_(grenz) impact a time circuit713 via a delay circuit 712, time constant T_(syn), which thereupongenerates a pulse, internal release pulse F_(fe) of a preset lengthfollowing the preset delay defined by the delay time T_(syn). The delaytime T_(syn) takes into account that upon resumption of the first systemvoltage a certain time is required until its own reference voltagegenerator can generate the system-synchronous reference voltage requiredfor normal operation of its own converter from the returning systemvoltage.

If, however, during a system malfunction, the system's own voltage dropsbelow the limit value, then from G₇₁₁ ="0" a pulse F_(de) (internalmalfunction warning pulse) is generated using assembly 74 which as itskey element contains a further time circuit 741, which passing throughan OR gate 702 activates the reset input of the malfunction memory 700with the corresponding malfunction signal F_(d) =1. The OR gate 701 and702 can also receive other suitable malfunction or release notificationsusing further inputs whereby other critical quantities of its own a.c.voltage system, e.g., impermissible changes of frequency or phase, aremonitored. The limit value warning device 711 and the other monitoringcircuits to detect and monitor its own a.c. voltage can be accommodatedthereby particularly in the reference voltage generator utilizing thesignals generated therein anyway. For example, the proper operation ofits own converter can thereby be monitored or, using a start command,the system can be brought on line from a standstill.

The signal status Q=1 set by the release signal F_(f) indicates not onlythe undisturbed status of its own station, but also of the entiresystem. Correspondingly, the signal status Q=1 indicates a malfunctionand is set by the malfunction signal or defect signal F_(d).

For that purpose the output of components 71 and 74 which monitor theirown station are logically combined with an external release pulse F_(ff)or an external malfunction warning pulse F_(df) using the OR gates 701and 702 to the release signal, F_(f), or to the malfunction signal,F_(d). These impulses F_(ff) and F_(df) are generated by the component73 or 72 integral to their own station and notify by monitoring theelectrical variables at the d.c. voltage connections of their ownstation that the other external station has respectively gone intonormal operation or a malfunction-reflecting emergency operation.Insofar as its own system is operating properly, the pulses F_(ff) andF_(df) thus initiate the suitable operation of their own stationdepending on the functioning or malfunctioning status of the otherstation.

An external malfunction, for example, always exists if in the externalstation a short circuit of the system or of the converter occurs andtherefore the d.c. current passes through this short circuit as faultcurrent. Said fault current is noticable in its own station after adelay reflecting the travel time of the HVDC transmission line in thatthe d.c. current i_(d) deviates substantially from the current set valuedefined for normal operation; the deviation i_(d) *-i_(d) exceedscritical values. This also occurs if given long-term malfunctions theline is dead and therefore the current at the d.c. voltage sideconnections of the station in question no longer reacts to the control.

For that reason, in component 72 detecting the external malfunction thedesign incorporates a subtraction circuit 721 to generate the differencei*_(d) -i_(d) in its own station as well as a connected rectifier 722. Aconnected bandpass filter sees to it that short-term power deviations,particularly those arising during the commutation times of its ownconverter, are suppressed in the same way as constant or relativelylong-term deviations. Said long-term deviations arise, for example, whenstarting up to normal operation before the HVDC transmission line cantransmit the complete d.c. current i*_(d) following a malfunction. Asuddenly appearing increase or drop of the d.c. current extending forseveral milliseconds is noticed at the output of filter 723 andregistered by a limit value warning device 734. Since said malfunctionalso arises if the local station itself is malfunctioning, the outputsignal of the limit value warning device 724 is only released to an ANDgate 725 if, for example, using the limit value warning device 711, thenormal status of the local station has been reported.

If, on the other hand, the external station has resumed normal operationafter eliminating an external malfunction, then this becomes noticablein the station in question after a delay due to the travel time of theHVDC transmission line by means of a sudden change of voltage and/orcurrent at the d.c. current connections. Therefore, for example, theinput d.c. voltage U_(d) or U_(dL) (for example U_(d) =U_(dA), U_(dL)=U_(dLA) for the station, A is differentiated using a differentiationcircuit 733 in order to generate subsequently an external release pulsein a limit value warning circuit 734 as soon as the voltage change hasexceeded a certain limit value. The output signal of this limit valuewarning circuit 734 can, in turn, be connected to an AND gate 732 ifassembly 71 reports a malfunction of its own system.

In this connection it can be advantageous to generate the requiredexternal release pulse F_(ff) also by a corresponding monitoring of thecurrent by elements 730, 731, 735.

A limit value warning circuit 734 can be designed in accordance withFIG. 12 to increase the reliability for the detection of resumedoperation in the external station. The changes in the electricalquantities (e.g., U_(dB)) brought about by a resumption of normaloperation by the external station (in the example of FIG. 12: thestation, A at the d.c. voltage connections of the station in question(in FIG. 12: the station B) indicate a typical curve function from whichpulse F_(ff) is derived.

FIG. 12 assumes that the line has become deactivated during amalfunction and converter 1A thus blocked or has been regulated to zerod.c. voltage α_(A) =90°. In order to recognize in the station, B, thatthe station, A, started normal operation, the commencement of rectifieroperation is introduced by a pulsating shift of control angle α_(A) attime t₁. At time t₂ a smooth running up of the control angle to anoperating value near to full-open control proceeds. This leads to thedepicted pattern of U_(dA) and i_(dA) with t₂, in particular beingpreset so that i_(da) =0 can be maintained.

With a delay reflecting the line travel time, a typical voltage patternU_(dB) appears in the station, B, whose differential in the station, B,monitoring device is monitored for possible exceeding of either apositive or a negative limit value. The limit value warning devices731.1 and 731.2 designed for that purpose therefore set a flipflop 731.3for a time period ΔT_(H) which is reset from the start of themalfunction, e.g., malfunction signal F_(dB). Circuit 731.4 monitors thetime period ΔT_(H) or its significance, for example, by entering theoutput signal of the flipflop monitor in a shift register whichgenerates an external release impulse F_(ff) if the output signal Φ_(D)shows the typical stored period ΔT_(H). The station, B, after a presetlag time then also resumes its normal operation by a smooth running upof the control angle α_(B) at time t'₂.

Similarly, further monitoring arrangements and logical combinations ofthe respective release signals or malfunction signals can be designed inorder to clearly handle any special operating cases. The design andinstallation of such logic circuits can be additionally installed at anytime by those skilled in the art using circuitry such as depicted inFIG. 11 as soon as it is detected in the course of investigating thepossible operating statuses and operating malfunctions, any conceivableambiguities in the generation of the external release signal, theexternal malfunction signal, the internal release signals, and theinternal malfunction signal.

The particularly simple monitoring device shown in FIG. 11 can besupplemented or expanded by further monitoring elements which permit amore precise recognition of the operating status of the externalstation, with the information obtained from the HVDC transmission lineconnections regarding the operating quantities of the external stationalso being usable to control the former station in normal or emergencyoperation.

This is demonstrated in FIG. 13 for the station, A, using a HVDCtransmission line whose converter, 1A and 1B are repectively connectedto the HVDC transmission line using converter reactance coils andfiltering circuits in accordance with FIG. 1. The HVDC transmission linevoltage itself is depicted in the replacement circuit as the sequentialconnection of series inductances L1 through LN and parallel capacitancesCO through CN.

In the station, A, the automatic control device can be recognized by thesummation point 42A and linearization device 40A, which aside from theoutput signal of the control variable controller 41A' to which can beconnected via a selector switch 61A' the fault indication voltage, i.e.by HVDC transmission line voltage U_(dLA) or a model value U_(dB) forthe fault indication voltage to function as the automatic controlvoltage U_(dAv).

The computing circuit to generate the model value U_(dB) is installedhere in the monitoring and programming circuitry 62A' as an additionalcomputing component 43A" which in part also has a monitoring function,as will be explained in FIG. 14. At the same time it supplies also amodel value i_(dB) for the HVDC transmission line current i_(dB) of theexternal station which serves as a replacement actual value for thecontrol variable controller 41A'. The corresponding set value i*_(dB) issupplied by the run-up function generator 66A which optionally, byselector switch 60A, is connected either to the output signal of asuperimposed controller 68A (e.g., for amplitude U_(A) of the a.c.voltage system, NA) designed for emergency operation, or the outputsignal of a superimposed controller 51 designed for normal operation.For controller 51, in particular, the same automatic control can bedesigned as shown in FIG. 7.

Computer component 43A" simulates the structure of the HVDC transmissionline as shown, for example, in the manner of FIG. 14. For that purposethe actual value of the converter output d.c. current i_(dA) is obtainedat the SRA block and supplied to the HVDC transmission line simulatingcircuit. The filter circuits of both stations are simulated by thesecircuits FA and FB.

Thereby model values i_(FA), U_(dLA), and U_(CN) =U_(dLB) for theelectrical values i_(fA), U_(dLA), and U_(dLB) of the HVDC transmissionline are generated using the designations of FIGS. 1 and 13. Only theconverter reactance coil LB has to be simulated at the integrator fromthe filter circuit of the station, B, which supplies, based on U_(dLB)and model value U_(dB), the model current i_(dB). To generate U_(dB),first using the subtraction point 75 one can subtract the ohmic voltagedrop r_(H).i_(dA), which corresponds to the resistance r_(H) of the HVDCtransmission line, from the model value U_(dLA). In addition, however,one has to consider that the external converter controls its HVDCtransmission line voltage U_(dB) by drawing current from the HVDCtransmission line. Said current i_(dB) is, however, determined by thecurrent i_(dA) impressed before the HVDC transmission line travel time.

Therefore the control differential i_(dA) -i_(dB) can be eliminated by areset controller 76 whose output signal corrects the U_(dB) andeventually ensures that the model current i_(dB) in normal operationcorresponds to the actual current i_(dB) of the foreign station.

A failure of the foreign converter can be as easily recognized by thedifferential U_(dLA) -U_(dLA) or the control differential i_(dA) -i_(dB)at the comparison circuit 721 of the monitoring device of FIG. 11. Insaid malfunction the d.c. voltage U_(dB) namely is short-circuited andthe HVDC transmission line discharges itself through the short circuit.Given an initially unchanged control angle α_(A) in the station inquestion, the current i_(dA) therefore rises following a lag determinedby the HVDC transmission line travel time. As the simulation in FIG. 14has, however, not yet taken said short circuit into account, thedifferential i_(dA) -i_(dB) reaches high values.

If in the model circuit of FIG. 14 following said malfunction warning,the voltage U_(dB) is now short-circuited by selector switch 77, thenthe malfunction situation of the short-circuited external station, i.e.,inverter failure, is also simulated. In this condition the model circuitis in a position to recognize from a renewed sudden change of i_(dA)-i_(dB) when the external converter resumes its normal operation and theHVDC transmission line impresses a voltage U_(dB) not equal to zero. Theselector switch 77 is then repositioned and once again the resumednormal operation is simulated.

In a model circuit in FIG. 14 one has therefore the first form of asystem observer which determines simulated values for the electricalquantities of the external station exclusively from the electricalquantities available in its own station. Remote signal lines are alsonot required in this arrangement, and the information regarding theoperating status of the external status is available following theshortest possible delay, namely the travel time of the HVDC transmissionline itself.

Naturally this principle, whereby the foreign station by simulating theHVDC transmission line and several circuitry components of the externalstation is monitored in the station in question, can be further improvedand modified if required. The advantage in this connection is that withi_(dB) and U_(dB) model values are available for the electricalquantities of the other station, both for normal as well asmalfunctioning operation.

Therefore, as already discussed, the model current i_(dB), inparticular, can be used as the substitute actual value for the currentcontroller 41A' in FIG. 13, which thereby permits control of the currentflowing through the short-circuited external station in the event of amalfunction.

This principle of system monitoring also permits recognition of the endof the malfunction in a malfunction situation and, depending upon thetype of malfunction, transition into normal operation with high powertransmission within a few milliseconds, e.g., 10 ms approximately.

TRANSITION TO NORMAL OPERATION: THYRISTOR SELECTION ANDSYSTEM-SYNCHRONOUS FIRING

In the normal operation described above the HVDC transmission line canalso be used due to its capability of rapid control of powertransmission to dampen balancing processes, to position the systems forreactive load, and for other tasks. For that reason it is advantageousfor users of such systems that following failure of the HVDCtransmission line normal operation resumes as soon as possible.

In this context various types of malfunctions must be differentiated.

If in the first station operating as the rectifier, the system, NA,breaks down or a converter malfunctions, then a short circuit of thed.c. voltage U_(dA) arises. If the HVDC transmission line has fullydischarged during the first station's breakdown via a short circuit lineor, for example, even via the converter of the second station, and ifthe second station is switched off to avoid an energy flow reversal,without, however, there being a malfunction either in the a.c. voltagesystem or in the converter in the second station, e.g., functioningstation, B, then following the end of the malfunction the task is to runup the currentless HVDC transmission line from the now recuperatedstation, A, when the system voltage U_(A) returns.

The same task is also at hand if after initial installation or aftermaintenance work the HVDC transmission line is placed into operation.

The invention also deals with the case in which following a rectifiermalfunction the HVDC transmission line has not been fully discharged.For example, the rectifier operation could have been interrupted onlyfor a short period or the converter 1A could have been bridged by abypass circuit without converter 1B being shut down. In this bypassoperation the HVDC transmission line serves only as a reactive load forthe system, NB, and remains charged. The current flowing through thebypass line functions as reactive current for the functioning station.

Such bypass operation has the advantage that the HVDC transmission linedoes not have to be run up from a fully discharged level, but thecurrent rather has to be raised only to the level designed for normalenergy transmission, thereby reducing the startup time. In addition,even during the malfunction, the HVDC transmission line can be used tostabilize the system in the functioning station.

The bypass circuit can thereby be closed over its own bypass switch(switch 80A in FIG. 13) of the station, A; preferrably, however, thebypass circuit is routed via thyristors lying in series of converter 1A,e.g., thyristors R⁺ ', R⁻ ', R^(+"), R^(-") of FIG. 1. In bothinstances, through U=0, the bypass circuit results in i_(dA) ≠0, U_(dLA)≠0, U_(dB) ≠0.

In this case, thus, one must take into account that the functioningstation, B, is current-conducting and that the increasing HVDCtransmission current arising during resumption of normal operation mustnot lead to any inverter failure in that station.

In the station, B, a drop of the a.c. voltage U_(B) results in a suddenrise of the current flowing into system, NB, with the currentcommutation exceeding the decisive maximum extinction angle, i.e., theprotective angle, for the inverter step limit. The inverter thereforefails and short-circuits the voltage U_(dA). Here as well the design canforesee shutdown of the converter 1A of the functioning station basedupon a malfunction warning so that the HVDC transmission line dischargesvia the short-circuited station, B.

Here as well, it is advantageous to continue operating the station, A,and recharge the HVDC transmission line with the station, B, beingshort-circuited by a bypass circuit. For this bypass circuit also abypass switch (80B shown in FIG. 13) or specific thyristors preselectedby programming and arranged in series in converter 1B can be used, ordirectly the thyristors of converter 1B which reflecting their phaseposition of the a.c. voltage system, NB, happen to conduct currentduring the malfunction and initiate the failure. Due to the alreadydescribed advantages of bypass operation a bypass circuit in thestation, B, itself could be desired, even if, depending upon the type ofmalfunction, no inverter failure arises necessarily or the operatorspermit a punctual shutdown of converter 1B.

Particularly in the event that the bypass circuit of the station, B, isrouted over the thyristors which during the failure happen to becurrent-conducting, the thyristors with which the system-synchronizednormal operation is to be resumed have to be dealt with depending uponthe system beat on the one hand, i.e., on the phase length of thereturning a.c. voltage U_(B), and depending upon the thyristors alreadyconducting current in bypass operation, on the other hand.

The required phase position for the system-synchronous startup of thea.c. voltage returning to the recuperated station can, however,initially only be incompletely detected by the reference voltagegenerator because this a.c. voltage usually has superimposed on it acombination of back-electromotive-force and harmonics which only declinegradually. The invention permits synchronization errors up to 30 degreesand permits resumption of normal operation in the recuperating stationas early as a few milliseconds afterwards in such a fashion that thefunctioning station can recognize this operating condition rapidly andfor its part respond by resuming normal operation as well.

For resumption of normal operation in accordance with this invention itis important that the recuperated station sends a signal over the HVDCtransmission line when the malfunction is corrected that is unmistakablyrecognizable in the functioning station. Preferrably the recuperatedstation impresses a voltage pulse into the HVDC transmission line.Since, however, the impressing of a voltage pulse is particularlydifficult after a malfunction in the inverter, it is desirable toinitially describe the impressing of the voltage pulse in station Bbased upon FIGS. 15 through 22, whereby the required mechanism in thestation, A, will be seen to be merely a simplification of the mechanismalready described for the station, B.

In the left-hand portion of FIG. 15 the already known configuration ofregulation and control mechanism 4B, the monitoring mechanism 64B andthe memory circuit 63B are shown. As this figure refers to a converter1B comprising component current converters in accordance with FIG. 1,the design incorporates two run-up function generators 66B' and 66B" inthe run-up function generator 66B already familiar from the 4B unit inFIG. 8. These two run-up function generators can be started at differenttimes by firing pulse release signals QIX and can be connected to theautomatic control device 42B following a time lag which can be set usinga delay circuit ZS.

The monitoring device 64B is only symbolically depicted by themalfunction memory 700 and the gates 701 and 725, whereby gate 701issues the release signal F_(fB) to the malfunction memory even in theevent of a start pulse start command such as one obtained from thememory circuit 63B being inputted.

To explain the circuit, we will start with the case in which the HVDCtransmission line is started up with such a start command from thede-energized condition in which all thyristors will be clamped by theclamping circuits. In the left-hand portion of FIG. 16 said clampedstatus is depicted. For T₀ the control angles α'_(B).=α_(B) '=90° arepreset corresponding to the zero output voltage of the converter. Thedrive unit STB' then supplies, for example, a system-synchronized firingcommand R⁺ 'α corresponding to α_(B) =90° which along with the startcommand is passed to an AND gate GU1 as shown in FIG. 15, generatingonly at time T_(0') an output signal FS₀. The firing command R⁺ 'α isselected from the firing command sequence S'α in accordance with theautomatic programmed operation mode specified for this case because thesystem-synchronized firing cycle of converter 1A' is to begin withthyristors R⁺ ' and S^(-').

The signal FS₀ is passed through an OR gate GO1 as a synchronous startuprelease signal FS on the dynamic input of a dynamic flipflop IZ. Thereit generates the firing pulse release signal QIZ=1, whereby the run-upfunction generator is triggered if there is no system malfunction, e.g.,if QB=0.

FIG. 16 also shows the firing commands R⁺ α^(') through S⁻ α' generatedby the drive unit STB' for the component current converter 1B' as wellas the commands R⁺ α" through S⁻ α" generated by STB" for the componentcurrent converter 1B", whereby to simplify the process it is assumedthat α_(B) '=α_(B) ". These firing commands generated by the drive unitsare, however, inhibited before time T₀, and thus shown only by brokenlines.

This inhibiting takes place in the clamping circuits BS', BS" triggeredby QIZ through AND gate GU2 as indicated in FIG. 17. By the coincidenceof the start signal and the firing command R⁺ α' at AND gate GU1 finallyQIZ=1 is set and the inhibiting of the firing commands removed so thatnow the firing commands can be passed through the correspondingamplifiers VR⁺ through VS⁻, thereby firing the thyristors R⁺ and S⁻ inthe respective component current converters.

These thyristor firings proceed reliably since the signal FS₀ at thesame time generates an impulse FZ₀ via a pulse generator IF₀ which viaOR gates GO3R⁺ and GO3S⁻ regulates the amplifiers VR⁺ and VS⁻. Thismeans that the firing pulse R⁺ ', in contrast to the firing command R⁺α' issued by the drive unit is extended by the crosshatched surfacemarked in FIG. 16, while the crosshatched surfaces for the firing pulsesS⁻ α', R⁺ α" and S⁻ α" show the pulses FZ₀ coupled over GO3R⁺ and GO3S⁻.

If a control angle α≦90 is preset for each relevant component currentconverter, then thyristors R⁺ and S⁻ fire and pull the voltage to theHVDC transmission line poles 2 and 3 (station A: U_(A) ≧0, station, B:U_(dB) ≦0 in accordance with the preset symbols) which thereupon by achange in the control angle can be run up to the operating valuedesigned for normal operation, which in the example of FIG. 16represents increasing the control angle to α_(B) =150° in the station,B.

This even applies if the preset control angle α in contrast to thereference voltage of the reference voltage generator due to an angleerror δ_(A) <30° of the reference voltage generator in contrast to theactual phase position of the system corresponds to an actual firingangle of a maximum of 120 degrees. Only at this limit angle does thecoupled system phase voltage U_(RS) which is to be coupled by R⁺, S⁻become negative and the coupled firing pulses R⁺, S⁻ have no impact.

This does not change even if at time T'₀ the thyristor combination R⁺,R⁻ and/or S⁺, S⁻ are already current-conducting, as is foreseen, forexample, for a bypass operation during a system malfunction. Since eventhen R⁺ and S⁻ will lead to the fact that the current commutates on thethyristor combination R⁺, S⁻.

If the bypass thyristor combination also includes the thyristors T⁺, T⁻,then the firing pulse leads to a current flowing via R⁺, T⁻, and thecoupled pulse S⁻ remains similarly ineffective as does the next T⁻ pulsewhich normally follows in the firing cycle on the R⁺ pulse at time T₀,in the firing pulse release signal QIZ=1. Only the subsequent firingpulse in the firing cycle, i.e., S⁺ will then bring about the normalcurrent conductance over S⁺, T⁻. The voltage, which until then wasconnected to the HVDC transmission line by the commencement of thesystem-synchronous normal operation, thus presents a voltage pulse whichcorresponds to a temporary rectifier operation with U_(dA) >>0 or elseU_(dB) <<0.

The circuit portion described thus far can therefore not only be usedfor startup of the de-energized HVDC transmission line using the startcommand, but also for the commencement of system-synchronized converteroperation in other operating conditions. If, for example, following aninternal malfunction the dynamic flipflop IZ is set to the output signalQIZ=0 over the reset input due to a corresponding output signal QB=1 ofthe malfunction memory 700, then the converter is inhibited. If given alonger malfunction the HVDC transmission line voltage has dissipated,then a release signal F_(fe) can now call up the signal status QB=0 sothat the dynamic flipflop IZ' can start up again with a startup commandand a consequently derived pulse FS₀ =1.

With an internal malfunction it can, however, also be designed so that abypass circuit closes when the current i_(dB) in the station, B,disspipates, in which, for example, the thyristors T⁺ and T⁻ are fired.P This thyristor firing takes place if the signal QB set by the internalmalfunction is coupled to memories SBT⁻ and SBT⁺ which in this bypassthyristor combination are set to 1, using a bypass supplemental pulse BZgenerated by a pulse shaper IF using the AND gate GU3T⁻ and GU3T⁺through a firing pulse VT⁻ and VT⁺. If now the bypass operation is endedby a start command, then when the firing command R⁺ α' appears, the gateGU1 again issues a signal FS₀ which clears the firing command inhibit bythe flipflop IZ in the clamping circuit BS of FIG. 17 and moreovercircuits the corresponding firing pulses to VR⁺ and VS⁻ using the pulsegenerator IF₀.

FIG. 18 shows the pattern of the thyristor currents iR⁺ through iT⁻,whereby initially during bypass operation iT⁺ and iT⁻ flow until due tothe described normal firing pulse sequence R⁺, T⁻, S⁺, R⁻ --initiated byFZ₀, the above-described current commutations discharge. Using as anexample a non-symmetrical system with strong harmonic signals when thesystem resumes operation, whose pattern is shown in detail in FIG. 22.FIG. 19 displays the resulting thryistor voltages UT⁺ and US⁺ forthyristors T⁺, S⁺ and the resulting HVDC transmission line voltages andcurrent conductance periods which are indicated by arrows. The impressedvoltage pulse is shown crosshatched and the reference polarity change isnegative corresponding to the transient rectifier operation of thestation, B.

Should the transient voltage pulse be more pronounced, it can easily becorrected by the control angle being shifted to rectifier operation inthe respective station, with this shift being prior to the running up ofthe control angle of FIG. 16 or being superimposed on the start of therunning up.

The same applies to the regulation of the second component currentconverter 1B", whereby simply the use of a lag circuit ZS which throughQB regulates the corresponding dynamic flipflop IZ at a certain timelag, assures that the startup of the second component current convertertakes place following a certain time lag. The design of the othercomponents marked by a double line is identical with the devicedescribed for the component current converter 1B' so that in thefollowing discussion both component current converters will be dealtwith and explained largely as one single converter.

With this variant it is assumed for transition from bypass operation tonormal operation that the HVDC transmission current i_(d) had dissipatedbefore closing the bypass circuit and that the bypass circuit was closedby a preprogrammed selection of thyristors connected in series. Thememories SBT⁺ and SBT⁻ can thereby be included in the memory circuit.

In the case of inverter failures, the series thyristors are alsocurrent-conducting; however, the thyristors which form the short circuitroute depends on the instantaneous phase position of the system duringthe failure. In order to rapidly transfer to bypass operation, thesethyristors can be retained as long as their combination is establishedand then in accordance with this combination those thyristors areselected among the converter thyristors that are suitable for restart ofnormal operation.

Memories SBR⁺ through SBR⁻ depicted in FIG. 17 serve for that purpose.They are also regulated by the firing commands Sα and determine at eachphase position of the system, thus at each moment within the cycle ofthe control command S, over which converter thyristors at that point aninverter failure could occur. This would at least be the respectivethyristor group which due to the exceeding of the maximum extinctionangle, in contrast to the normal commutation sequence, still carriescurrent and the thyristor groups connected in series thereto.

FIG. 20 assumes that the internal malfunction warning pulse triggered bythe failure occurs before the drive unit issues the next firing commandS⁺ (time T₁,) so that lastly thyristor T⁻ was regulated with thyristorR⁺ conducting. Therefore a memory SBR⁻ which can be reset by S⁺ α is setto the value 1 using the command T⁻ α and indicates that thyristor R⁺ isinvolved in the inverter short circuit. While by QIZ=0 at GU2 theregular command S⁺, generated by the drive unit is inhibited, at GU3R⁻the bypass supplemental pulse, shown marked by crosshatching in FIG. 20,is given to thyristor R⁻ via memory SBR⁻ and the command BZ. Thisthyristor R⁻, along with thyristor R⁺, which is not deactivated, nowbrings about a safe closing of bypass route R⁺, R⁻.

If the failure arises after another firing command, then memories SBR⁺--and gates GU3R⁺ --bring about the relevant selection, storage andregulation of the possible bypass thyristor combination. The design can,however, also foresee, particularly in the station, A, not to make thebypass thyristor selection so dependent upon the operation, but routedalways to establish a preprogrammed combination, as required several orall converter thyristor groups in series, in the memory circuit and tocouple them in the event of a short circuit by regulating gates GU3 orGO2.

To generate the desired voltage pulse when proceeding according to FIG.16, it is, however, necessary to select the required initial cycle inaccordance with the preset and operation-dependent bypass thyristorcombination to commence the system-synchronous operation. For bypassthyristors R⁺, R⁻ the system-synchronous firing should now start by acommutation of thyristor S⁺ or S⁻ which is the function of the selectorcircuit, AS, of FIG. 15.

Therefore, in accordance with FIG. 21, a pulse, e.g., pulse S⁺, S⁻, isalways generated in the sequence SZ from the firing pulses obtainedfollowing the interlock GO2 and issued regardless of whether the systemis in normal or bypass operation which is allocated (OR gate GO4R, GO4S,GO4T) to two series thyristor with one potential thyristor forming abypass pair. This potential bypass combination is clamped by the ANDgate GU4R . . . GU4T until in the event of a malfunction using thebypass supplemental pulse BZ the bypass thyristor firing actually occursand the bypass combination is stored in the memories SQ_(R). . . SQ_(T)which can always be reset at the end of a malfunction by the firingpulse release signal QIZ.

While thus the memories SB_(R) ⁺. . . SB_(T) ⁻ of the clamping circuitsalways determine the possible bypass thyristors from the firing commandsequence of the drive unit and in case of malfunctions bring about theirfiring, the gates GO4 select from the actual firing pulses issued thepossible bypass thyristors which are only stored in case of amalfunction.

Further OR gates GO5S, GO5R, GO5T unite the firing commands, e.g., forbypass combination R⁺, R⁻, thus the firing command combination S⁺, S⁻contained in the firing sequence Sα and required always to decommutatethe current from one of the selected bypass thyristors. These firingcommands then, by coincidence with the stored combination determined bygates GU5S, GU5R, GU5T, lead to a synchronized signal FSα.

The OR gate GO1 then issues as the startup release signal thesynchronized signal FS instead of the previously discussed start signal"start" which in flipflop IZ generates the firing pulse release signalQIZ if IZ is released after the end of the system malfunction (QB=0).

Thereby the desired startup of the system-synchronous normal operationstarts by itself after the termination of the system malfunction.

To summarize, FIG. 20 thus shows the following: In normal operationsignals QR⁺ through QS⁻ always indicate a thyristor which in case of acommutation malfunction could be used during the next firing as thebypass thyristor and should be fired by the bypass supplemental pulseFZ₀ in order to close the bypass. The system malfunction induces a dropin the system voltage U_(B) at time T₁ below a preset limit value whichvia QIZ=0 leads to the inhibiting of the normal firing command Sα or Zαand to the coupling of the crosshatched bypass supplemental pulse to R⁻.During the malfunction, therefore, only the thyristor combination R⁺, R⁻is current-conducting and this thyristor combination is stored in theselector circuit. After the system voltage resumes, at time T₂, and therelease signal occurs, also at time T₂, setting the malfunction memoryto Q_(B) =1, memories SQR, SQS, SQT of the selector circuit choose athyristor to resume normal operation and the selector circuit thengenerates the startup release signal if the system-synchronous converteroperated by the control angle issues the firing command corresponding tothis selected thyristor. The normal firing cycle is then released by thestartup release signal FS commencing with the firing of this thyristorand leading to the decommutation of the current from one of the bypassthyristors.

The a.c. voltage system starts, as FIG. 20 for the phase voltage(UB)_(R) and as the voltage amplitude UB indicates, very irregularly.The malfunction memory supplies a signal QB=1 only after a certain timerequired by the reference voltage generator to determine a relativelyreliable phase position of the system. A rapidly working referencevoltage generator is described in the German patent application No. P 3346 291.7 and assures that thereby the phased position of the system isdetermined down to less than a 30 degree precision level. A lag circuitcontained in the pulse generator 713 in FIG. 11 can assure that theself-release pulse Ffe in the station, B, and thus the release signaland QB=1 are only generated after the minimal time required forsynchronization.

FIG. 22 depicts the overlaying of the sine-shaped fundamental frequencyby counter voltage and harmonic oscillations as occurs typically forreturning connected phase voltages USR, UTS, URT of the a.c. voltagesystem. S* shows a hypothetical set of firing pulses associated with thezero degree of modulation, i.e., control α_(B) =90, and the fundamentalfrequency of the recovering system.

This set S*α is hypothetical, i.e., cannot be realized since thefundamental freuqency at time T₂ cannot yet be detected. Rather thereference voltage generator generates a reference voltage U_(Bsyn)associated with the similarly hypothetical firing pulse sequence S⁺ G,R⁻ G, etc. at the hypothetical control angle α_(B) =90°. This pulsesequence is also only hypothetical because in order to implement theprocedure according to this invention, no constant control angle α_(B)=90° is preset. However, a comparison of both hypothetical pulses S⁺ *and S⁺ G shows that at time T₃ a reference voltage U_(Bsyn) can deviatefrom the (undetectable) fundamental frequency by a phase differencewhich can reach up to 30 degrees. Only at time T₄ (typically T₄ -T₃ =8msec) has the reference voltage generator built up to a point that thereference voltage U_(Bsyn) is practically synchronous with thefundamental frequency of the system, which still has superimposed on itharmonic oscillations, and the equation R.sup. - α*=R⁻ G applies.

In order to resume normal operation as soon as possible after the systemresumes, the design does not have a provision to wait till time T₄ forthe release of the firing pulse. Rather, it suffices if thesynchronization error of the reference voltage is less than 30 degrees,i.e., the firing pulses are released already as early as time T₃.

FIG. 22 assumes that for the resumption of normal operation with thecontrol angle (α_(B) =90°) (i.e., control angle at initial setting of 0corresponding to a d.c. voltage set value U_(dB) *=0 the system canbegin and run up to a maximum value α_(B) =α_(max) within approximatelytwo system cycles which corresponds to the nominal value projected fornormal operation. Consequently thyristor S⁺ is fired at time T₃ by thefiring pulse S⁺ α=S⁺ G, while at approximately time T₄ the actual firingcommand R⁻ α, in contrast to the hypothetical command appertaining to R⁻G, α_(B) =90° is displaced by (α_(max) -90°)/2. At time T₅, α_(max)≐150° is attained.

TRANSITION TO NORMAL OPERATION; PRESETTING THE CONTROL ANGLE

In accordance with the previous explanations it is thus possible thatafter a malfunction has ended, one station issues a pulse to the HVDCtransmission line, particularly a voltage pulse, which can be detectedby the other side and initiates an internal release pulse whereby normaloperation is resumed in that station as well. The arrangement shown inFIG. 15 as hardware, can also be partially realized in software, andincorporated with the memory circuit 63B. The memory circuit 63Bbasically only has to determine which mode of emergency operation, e.g.,bypass operation or shutdown of the line, are to be selected for theparticular malfunction in question. The methods described below for thevarious types of emergency operating modes for resuming normal operationwill provide those skilled in the art with adequate information as tohow the individual signals of the monitoring device are to be combinedin the memory circuit for each individual case in order to execute therelevant startup program in conjunction with the design of the stationsshown in FIGS. 8, 9, 10 and 13.

In the following discussion, that particular station whose malfunctionhas caused an interruption of normal operation will always be designatedas "the former station." In case of a rectifier malfunction this wouldbe the station, A, and in case of an inverter malfunction it would bethe station, B. In this context, depending upon the energy flowdirection chosen for normal operation, the design can foresee that thestation arranged at one end of the HVDC transmission line once plays therole of the station, A, and another time the role of the station, B,since the task and design of the monitoring device and the memorycircuit both for rectifier as well as for inverter operation are largelythe same so that both stations are designed to be as identical aspossible.

After overcoming the malfunction in the former station, thecorresponding monitoring station, designated as "the former stationmonitoring unit"; generates an internal release pulse which in thisformer station produces the release signal, described as the "leadingrelease signal", whereby the startup is initiated.

This startup is then obtained after the HVDC transmission line traveltime at the d.c. voltage side connections of the latter, undisturbed,station or of the latter converter, thereby initiating in the othermonitoring device an external release pulse which produces thederivative release signal.

The basic concept for startup is then that when a non-problematic or"recuperated" status of the previously malfunctioning former station isestablished, a leading release signal is generated, initiating theresumption of system-synchronous normal operation in the former stationconverter. In the latter station, on the other hand, the effects of thischange at the d.c. voltage side connections are converted into aderivative release signal which in that station similarly initiates thesystem-synchronous normal operation which thus begins after the shortestpossible delay upon recuperation of the former station. Remote signallines are not required.

This assumes on the one hand that the electrical processes brought aboutin the former station are clearly recognized in the latter station and,on the other hand, that the processes in both stations must becoordinated with each other.

Both prerequisites are met by having the HVDC transmission line in theformer station be impressed initially by a voltage surge with thecontrol angle of the former station converter being run up in accordancewith a preprogrammed run-up function approximately to the valueprojected for normal operation whereby, as shown in FIG. 22, the voltagesurge is either necessarily generated by the run-up of the control angleor, if required by the situation, can be generated by a separate andtemporary pulsating shift of the control angle. The control angle isthen similarly run up in the latter station starting from an initialvalue of approximately zero corresponding to an angle of approximately90 degrees in accordance with a preprogrammed run-up function to thevalue projected for normal operation.

Both run-up functions are coordinated with each other. Preferrably theirrun-up time would be approximately two a.c. voltage cycles.

During the malfunction the malfunctioning converter of the formerstation is either totally shut down or short-circuited using the bypassroute. Thereby during emergency operation the model fault indicationvoltage in the automatic control device in the control channel of theformer station drive unit does not have to be switched on. On the otherhand, the relevant control angle at the end of the run-up cycle has toattain the control angle projected value projected for steady stateoperation at the end of the run-up cycle or at least attain this steadystate value, if possible without substantial jumps. If therefore asmooth run-up is generated by a run-up generator at the input for thepreset angle, then this preset angle which generally is generated by asuperimposed control quantity controller, e.g., the current controller41A or the extinction angle controller 41B, would have to be retractedto the extent that now the automatic control model fault indicationquantity determines the control angle.

The preferred arrangement is therefore that the run-up generator 66Agenerating the run-up function be located, at least in the station, A,as shown in FIG. 10, at the entry to the automatic control voltage ofthe automatic control device in order to connect using the selectorswitch 61A the model fault indication quantity of the output signal ofthe run-up generator as a substitute value during emergency operationand the run-up cycle. Only after attaining the run-up function finalvalue does the system switch over to the measured value U_(dLA), or thecorresponding model value U_(dB), by switching the selector switch 61A.Once steady state normal operation has been resumed, then also thepreset angle will be obtained from the current controller 41A asdesigned for normal operation, but which can be deactivated during themalfunction.

In the station, B, the model fault indication quantity serves to set theextinction angle and to maintain the inverter step limit. If duringenergency operation the system dispenses with the automatic control withthe corresponding automatic control angle α_(Bv), no danger arisesregarding any possible inverter failure, since even in case of bypassoperation the control angle is still far from the inverter step limit.This also applies, if after a rectifier malfunction, the station, A,starts to increase the current. If therefore the station, B, resumesinverter operation after a derivative release signal, then the presetrun-up function in that station generates a controlled model substitutevalue for the model fault indication quantity which can also replace themodel fault indication quantity obtained from the actual inductive d.c.voltage drop and lead to a limitation of the extinction angle. Thereforehere as well the run-up generator can be arranged at the automaticcontrol angle input of the automatic control device and be switched bythe selector switch between the regulated preset model value supplied bythe generator and the model fault indication quantity whose value iscomputed from the inductive d.c. voltage drop if or when the run-upvalue has approximately been obtained.

The two run-up functions thus provide substitute values for the relevantautomatic control quantities. It thus seems initially necessary thatboth run-up functions coincide, i.e. are identical in terms of theircurve and plateau level. Such rigid coordination of both run-upfunctions is, however, not required. For example, the run-up end valueof the run-up generator of the station, A, can generally be somewhathigher and lead to a HVDC transmission current which could, with regardto the preset control angle in the run-up function of the station, B,lead to an extinction angle beyond the inverter step limit. Theextinction angle control of the station B is, however, in a position toretract the control angle α_(B) in accordance with the inverter steplimit.

For the various types of emergency operation the following possiblevariants resulted:

(a) Startup from a deactivated line:

In this case, after the malfunction arises, both converters areinhibited whereby the discharge of the line, e.g., during a shortcircuit of the voltage U_(dB), could be handled by the short circuit inthe station itself or also, for example, given a rectifier malfunctionin the station, A, by a forced temporary firing angle shift in the otherfunctioning station.

When the malfunctioning system goes back on line, the correnpondinginternal release pulse in the former monitoring station initiates theleading release signal. The resuming normal operation in the formerstation initiates the derived release signal in the latter station,which there, too, initiates the startup in accordance with thepreprogrammed run-up function. Both run-up functions start at an initialvalve of approximately zero, i.e., control angle approximately 90degrees.

As a supplement, the design can foresee that the control angle of theformer station is determined from the run-up function and a temporaryadditional shift in the direction of the rectifier wide-open control,wherein this additional shift can either be set before or superimposedupon the run-up function.

Given a malfunction of the inverter operation in the station, B, thismeans that the already described negative voltage pulse, which isimpressed when resuming normal operation, is reinforced. This simplifiesboth the generation of the derived release pulse as well as the start ofrectifier operation in the station, A. Even after a malfunction ofrectifier operation in the station, A, such a pulse is advantageoussince it means that current i_(dA) and voltage U_(dA) rise rapidly andcharge up the HVDC transmission line so that both the run-up of thepower transmission to the value projected for steady state normaloperation can take place more rapidly and the derived release signal ofthe station, B, can be generated more easily.

(b) Emergency operation with bypass circuit:

In case of longer malfunctions the operation can proceed using thebypass circuit, whereby in the malfunctioning former station, when theinternal malfunction warning pulse arises, the system-synchronous firingcommands of the converter in the former station are inhibited and thebypass circuit closed. As already explained, this bypass circuit canbest be closed by the firing of selected converter thyristors in series.In the latter station using the external malfunction warning pulse, thecontrol angle is shifted to such an extent to rectifier operation asrequired to feed bypass current into the HVDC transmission line to meetthe requirements of the latter system.

The transition from this bypass operation to nornmal operation takesplace when the system of this former station has recuperated andgenerates the leading release signal which initiates resumption ofsystem-synchronous converter operation with the control angle running upto the proper operating degree of control. Simultaneously the bypasscircuit is extinguished which generally no longer requires any specialmeasures to be taken if no specific bypass switch or bypass thyristor,but rather a series of converter thyristors required for normaloperation, were being utilized.

In the latter station the derived release signal then initiatesresumption of normal operation whereby the control angle runs up to thecontrol angle steady state value in accordance with the run-up functionfrom the control angle determined by the bypass current.

If the bypass operation resulted from a malfunction of the station, A,then it is not necessary to undertake an internal pulse shapingdisplacement of the control angle at the start of the run-up cycle, andthe system-synchronous firing at the start of the run-up cycle, i.e.,the introduction of normal rectifier operation, poses no problems.Rather, in accordance with FIG. 16, the rectifier operation can beginwith the synchronous operation of specific preprogrammed thyristors.Even the selection of the bypass thyristors is preferrably programmedwhereby the bypass current can be routed not just over one bypasscircuit formed by thyristors in series, but rather over several or evenall the converter thyristors. This is possible and under givencircumstances advantageous if, for example, suitable measures are takenin the station, A, to ensure that after a malfunction the returningsystem, NA, will not immediately discharge again via the bypass circuit.

During bypass operation wherein the bypass circuit is closed bythyristors in series of the converter of the station, B, the selectionof the bypass thyristors for the duration of the malfunction is stored.In accordance with this stored thyristor combination one of thethyristors is selected by the selector circuit as described in FIG. 15for resumption of the system-synchronous normal operation. In the cycleof the system-synchronous firing commands this thyristor thencorresponds to the decommutation of the current from one of the bypassthyristors.

FLUCTUATION BETWEEN NORMAL OPERATION AND EMERGENCY OPERATION

The normal operation described permits, in accordance with the automaticcontrol principle, rapid control of the HVDC transmission line without,however, being able to exclude a malfunction altogether. The principleof programmed run-up without the use of initiating remote controlsignals permits transition into normal operation both given adeactivated HVDC transmission line as well as in case of bypassoperation within very short startup times. The bypass operationprinciple described below thus permits, given a malfunction of one ofthe two stations, the operation of the HVDC transmission line evenduring the malfunction as the reactive load for the a.c. voltage systemof the normally functioning station and to be regulated or controlled inaccordance with the requirements of the normally operating system.

Each of these three principles: automatic control, programmed run-up,and bypass operation can, in conjunction with other operating proceduresof a HVDC transmission line, be advantageously used separately or inconjunction with another one of these principles. The combination of allthree principles described below permits rapid control of the HVDCtransmission line, both in normal as well as in emergency operation, inorder thereby, for example, to dampen sub-synchronous resonanceoccurances or other dynamic balancing processes in the systems, tominimize the economic effects of a malfunction, particularly of avoltage drop in one of the a.c. voltage systems and always to be able tocomplete the transition between normal operation, bypass operation andoccasional, unavoidable operation with deactivation of the HVDCtransmission line, within the shortest possible time.

The principle of bypass operation, including the transition into bypassoperation in case of a malfunction, is explained in further detail basedupon the signal patterns in FIGS. 26 and 27 and the design models of thetwo stations shown in FIGS. 9 and 10. Aside from this bypass operationit could be necessary, depending upon the type of malfunction, todeactivate the HVDC transmission line during the malfunction; this typeof emergency operation is shown in FIGS. 24 and 25. To resume normaloperation, it could be desirable, as already explained, both for a HVDCtransmission over long distance as well as for a short coupling that,for example, the transition from bypass operation to normal operation bedesigned so that the malfunctioning system primarily carries reactivecurrent when the voltage resumes.

This transition can be influenced not only by the d.c. current itselfwhich has to be run up, but also from the number of available componentcurrent converters simultaneously participating in the transition.Thereby the possibility arises of decoupling the desired gradations inthe d.c. current rise from the rise of the system reactive current indiscrete steps so that thereby the requirements made of the system andof the current control can be handled separately to a certain extent inaccordance with the number of separately connected component currentconverters.

This can be done in accordance with FIG. 15 by having the timer circuitZS, which releases the generation of the firing pulse release signal todisable the clamping switch BS" of the component current converter IB"at the reset input of the dynamic flipflop, delay the start of thesystem-synchronous converter operation against the component currentconverter 1B'. The pulse release of the individual component currentconverters selected and dependent upon the combination of bypassthyristors, thus proceeds in steps including time delays, whereby theupswing of the system-reactive load also proceeds in steps.

In the following discussion, a run-up function is selected for theincrease of the control angle in general which drives the control anglefrom a value which, if necessary, deviates slightly from 90 degrees in alinear fashion till it attains the steady state condition. Althoughinitially it seems essential that the two run-up functions, which asregulated values substitute for the preset operating model value of themodel fault indication voltage when transitioning into normal operation,be practically identical, such tight coordination of both run-upfunctions is not in fact required. It can in fact be advantageous topreset the two run-up functions to run in opposite directions insofaras, for example after a malfunction in the station, B, the control angleof the rectifier can be run up initially with a larger slope and laterwith a declining slope, while the inverter control angle initiallymounts slowly and later mounts more rapidly.

Thereby the functioning station is to be recharged by active current asquickly as possible so that the HVDC transmission line can be rechargedquickly. For the initially malfunctioning station whose returningvoltage often displays marked switch-on peaks, the HVDC transmissionline initially functions as a voltage reducing reactive load while atthe same time attaining reliable commutation.

(a) Example with different run-up functions

FIG. 23 shows this run-up variant using as an example a systemshortcircuit in system, NB, and a bypass operation during themalfunction.

At time t₁₀ voltage U_(B) breaks down and the limit value warning outputsignal G711B indicates a shortfall of the limit voltage U_(Bgrenz) andsets the controlling malfunction signal QB=0. This initiates acommutation lock in the clamping device of converter 1B, whereby allregular firing commands of the drive unit are suppressed whilesimultaneously the bypass circuit is fired by a supplemental bypasspulse; this condition is symbolized by a control angle α_(B) =90° inaccordance with the output d.c. voltage U_(dB) =0.

Using the bypass circuit, the charge in the HVDC transmission line andthe filter of the station, B, is reversed as can be recognized by acurrent flow i_(dB) (which often does not even attain the conditioni_(dB) =0) whose extent is exaggerated in the depiction of FIG. 23, andthe oscillation of voltage U_(dLB) at the end of the converter reactancecoil LB facing away from the converter. The voltage U_(dB) which duringnormal operation, for example, was held constant at an operating valueU*_(dBo) and was short-circuited during the current surge takes on anegative value, if the current i_(dB) due to the charge reversal becomeszero and the bypass thyristors extinguish. The control quantity control41B, as shown in FIG. 9, already made line-ineffective due to thedisabling of commutation and which would only display irregular valuesduring the malfunction by a defined value Δx_(B) =0.

Similarly, switch 61B can be switched to the temporarily inactive run-upfunction generator 66B at the same time or after the appearance of themalfunction signal QB=0 from the circuit 43B' which computes the modelfault indication quantity from actual values i_(dB) and U_(B).

The current i_(dA) of the station, A, which during normal operation iscontrolled to the operating set value i*_(dAo), i.e. the normal value,similarly shows, following the transmission travel time, an increasewherein the difference i*_(dA) -i_(dA) attains a limit value which leadsto the derived malfunction signal QA=0(t'₁₀). Thereby the control anglewill no longer be present in a normal fashion wherein the correspondingnominal value α*_(Ao) (e.g., α*_(Ao) ≐150°) respectively. Δx*_(A) isgenerated by current controller 41A, as shown in FIG. 10, adjusting thei*_(dAo) parameter and whereby the fault indication voltage U_(dAv) isgenerated by a measurement circuit as the automatic control quantityU_(dAv). Rather by reversing the switch 60A at time t₁₀ the systemswitches from the set value for the operating current to a new set valuesupplied by a bypass controller, i.e., a controller for the voltageU'_(A).

As in the case under consideration only a bridging circuit of converterthyristor groups in series was selected and fired as the bypass circuitand as this bridging circuit has to be capable of bearing permanentcurrent, its thermal load bearing capability is lower than for normalsystem-synchronous operation. Therefore the bypass current set valuesupplied by the bypass controller is throttled back in contrast to thenominal value i*_(dAo).

The control angle supplied largely by the bypass controller 68A issymbolically assumed to be a constant in FIG. 23. Until the end of thebypass operation it can be pre-regulated by the measured voltage U_(dLA)or even by the substitute U_(dLA) or U_(dB) generated according to FIG.14; according to FIG. 23, at time t'₁₂ the automatic control is switchedonto the initially inactive run-up generator 66A in order to prepare fora normal start.

The d.c. current i_(dbypass) now supplied by the functioning the stationA, flows, after the voltage U_(dB) passes to the zero point at time t₁₁-over the bypass thyristors until voltage U_(B) returns at time t₂₀possibly with substantial voltage spikes.

The limit value warning signal G711B supplies at a delay T_(syn) inaccordance with the synchronizing time of the reference voltagegenerator the leading release signal QB=1 used to start the componentcurrent converter 1B' with control angle α_(B) '≐90°, whereby thecomponent current converter 1B' is shifted at a low increment of, forexample, 10 degrees into converter operation. Following a delay T_(ZS)supplied by the timer circuit ZS, as shown in FIG. 15, the componentcurrent converter 1B" is also released.

Thereby initially, as already discussed, a negative voltage timewaveform arises in U_(dB) and the system, NB, is charged with reactivecurrent which leads to the desired voltage decline and a reduction ofvoltage spikes in U_(B).

As the component current converter is switched on after a delay, thereactive load also rises in steps. Preferrably the control angle α'_(B)of the component current converter started first is left in the areanear 90 degrees by having the HVDC transmission current functionprimarily as a reactive load, and is run up to the operating inverterdegree modulation only after all the other component current converterscommute fully. Thereby, as FIG. 15 indicates, a distinct run-up functioncan be selected for each component current converter. According to FIG.23, however, one single run-up function generator ususally suffices forα_(B), whereby the slope of α' initially impacts only on 1B' and startsat a low increment to continue at a higher increment to the nominalvalue α_(Bo) only after time t₂₀ +T_(ZS) after the last componentcurrent converter is started with (α"_(Bo) =α_(B)).

The voltage pulses impressed by the station, B, lead following thetransmission line travel time Tts to a motion of U_(dA), in other words,U_(dLA) ; and i_(dA), in other words, i_(dLA), which is detected in thestation, A, and initiates the derived release signal QA=1 at t'₂₀. Therethe control angle α_(A) is run up to α_(A) ≐0, with the incrementinitially being preferrably large and later being throttled down. Sinceinitially only the fault voltage U_(dB), in other words U_(dLA),impressed by the initially slight increment of α_(B) impacts on thestation, A, i_(dA) rises rapidly to the nominal value i_(dAo), andconverter 1A is soon in a range in which the current i_(dA) loading thesystem NA, is primarily active current.

During run-up switch 60A is reversed to the operating set value i*_(dA),whereby the bypass controller is out of action. After the run-upfunction starts in FIG. 23 at time t₂₁ the control quantity controller41B is activated and approximately at the end of the run-up function, atthe preset times t₂₂, t'₂₂ controlled by the lag circuit VZ, theoperating automatic control signal is always connected so that normaloperation can resume.

In normal operation thus both stations are released by the releasesignal QA=1, QB=1, while switch 61A and 61B are always positioned by thelast firing pulse release signal QIZ=1 so that the control angles α_(A)and α_(B) are determined in the automatic control device by the additionof the projected model fault indication quantity and the respectivepreset angle, i.e., the angles determined by the quantities Δx_(B) andΔx_(A) shown in FIGS. 9 and 10, for normal operation. Similarly,reversing switches 60A and 60B are held in a position whereby the presetangle α_(A) *, and respectively, γ_(B) * is determined by the controlquantity control projected for normal operation. For example, inaccordance with FIG. 7, to generate X_(A), the set value i*_(dA) of thecurrent controller 41A in the station, A, is supplied by a superimposedcontroller such as active power controller 51, shown in FIG. 7, inaccordance with the active power balance or other requirements of theundisturbed system, NA. In the station, B, Δ X_(B) is supplied inaccordance with a set value, γ*, supplied either by an extinction anglecontroller, such as 41B of FIG. 5 a reactive current controller, 41B',or voltage regulator, 41B", with γ* derivable in the computer circuit 47from the system requirement for reactive current or constant voltage.

Since for maintenance on the HVDC transmission line itself it might benecessary following a malfunction to deactivate the HVDC transmissionline in terms of current and voltage, i.e., not undertaking any bypassoperation, this situation will be considered first.

(b) Examples with Transmission Line Deactivated:

In a typical sequence as shown by FIG. 24 the case in which the formerstation, B, at time T₁₀ faces a short circuit of system, NB, i.e., abreakdown of voltage U_(B), is considered first. In the monitoringstation the limit value alarm 711 issues the corresponding signal G711B.The HVDC transmission line, the converter reactance coil, LB, as well asthe filter elements CFB and LFB discharge via the short circuit so thatthe currents i_(dB), in other words, i_(dLB), initially rise, leading toan inverter failure. At the same time, the malfunction signal QB=0 isset and the firing pulses of converter 1B are inhibited as indicated inFIG. 24 by the control angle: α_(B) =90°.

Thereby the voltage U_(dB) drops and the HVDC transmission line currentextinguishes.

In the station, A, following the travel time Tts determined by the HVDCtransmission line flow time there is a similar increase in the HVDCtransmission line current i_(dA), in other words i_(dLA), which over theexternal malfunction warning pulse produces a malfunction signal andleads to QA=0 at time t_(10'). Thereby this other station, A, is alsoshut down: α_(A) =90.

FIG. 24 assumes that due to a short circuit on the side of the station,B, the HVDC transmission line has been practically completely dischargedwith the exception of slight oscillations in the HVDC transmission line,which is now locked on both sides. In this context it can be ofparticular advantage if with QA=0 the other converter 1A is notimmediately locked, but rather the HVDC transmission line current andvoltage dissipate fully down to the value zero over the controller ofthe station, A. Such a procedure is particularly advantageous if, forexample, following a short-term interruption the HVDC transmission lineis to be started up as rapidly as possible from the deactivatedposition.

In this restart, as soon as the voltage U_(B) has attained a specificlimit value, i.e., time point t₂₀, and the limit value alarm 711 hasactivated in the one station following a certain lag time T_(syn) whichis necessary to synchronize the reference voltage generator as well asto generate the firing pulse release signal QIZ, the run-up functiongenerator 66B is activated and the control angle α_(B) run up smoothly.

In order to then impress the HVDC transmission line with a definedvoltage pulse which can release an external release pulse in the otherstation, A, it is preferably foreseen to add an additional pulse Δα tothe smooth run-up of the control angle α_(B) or to overlap said pulse,by means of which the converter 1B is temporarily operated in rectifieroperation. FIG. 24 shows that the HVDC transmission line current i_(dB)is thus rapidly excited and the HVDC transmission line voltage U_(dB)becomes temporarily negative.

As a consequence, again at time t₂₀ determined by the HVDC transmissionline travel time, a negative voltage vs. time waveform in the thus farlocked station, A, and a signal QA=1 result, with which the run-upfunction generator inactivated during malfunction is activated there andthe control angle α_(A) is run up smoothly.

In both stations during the malfunction inactivated run-up functiongenerators supply during the run-up initially a substitute value for themodel fault quantity, although one can recognize from the pattern of thecontrol angles α_(A) and α_(B) that the smooth preset value supplied bythe run-up function during the run-up is replaced by the pulsating modelfault quantity as soon as the reversing switches 61A and 61B and alsoswitches 60A and 60B are reversed and the undisrupted normal operationis resumed.

FIG. 25 depicts the case of a malfunction in the station, A. Thisassumes that in the system, NA, at the time t₁₀ only one of the phasevoltages fails so that the voltage amplitude U_(A) of this system showsa pulsating pattern. In this case the limit value alarm 711 alsosupplies a pulsating signal to one monitoring device which, however, isonly shown by the broken line in FIG. 25 because, for example, the timeconstant of pulse shaper 713 can be set so that a constant signalsuppressing these oscillations can be generated. In the case alreadymentioned, wherein no specific limit value alarm is designed to form thesignal G711B, but the pulse generation is rather handled by thereference voltage generator which handles all the fluctuations of thea.c. voltage system anyway, the relevant constant signal for the totalextent of the malfunction can be generated there easily.

In general, for such a malfunction another type of emergency operationis designed which is not the subject of this invention. The strategydesigned for a malfunction of the rectifier with deactiviation of theHVDC transmission line will be explained, however, for this case aswell.

The inhibiting of the firing pulses of converter 1A generated by QA=0brings about the extinction of the HVDC current i_(dA), althoughadmittedly there is no necessary immediate discharge of the HVDCtransmission line. Such a discharge can be forced, as already explainedin connection with FIG. 24, by a temporary firing angle shift in theundisturbed other station, B, which, however, is dispensed with in theexample of FIG. 25. Rather, the firing pulses of converter 1B in thestation, B, are inhibited following the lag time Tts by the collapsingHVDC transmission line current i_(dB) or HVDC transmission line voltageU_(dB) so that there, too, the current is discharged and the convertershut down. The HVDC transmission line then no longer conveys current,but still conveys voltage.

In such a case it is often not necessary following the return of thesystem, NA, i.e., time T₂₀, to begin the transition to normal operationwith a temporary supplemental pulse Δα on the control angle. Thisapplies, in particular, if the HVDC transmission line still retains apositive residual voltage as shown in FIG. 25, which when firing thethyristors in 1B, generates a current flow. The control angle can thenbe run up smoothly from approximately α_(A) =90°.

In the situations of FIGS. 24 and 25 the design foresees in case ofemergency operation during a malfunction a shutdown or deactivation ofthe HVDC transmission line at least to the extent that the HVDCtransmission line current is equal to zero which is attained by havingthe HVDC transmission line separated from the a.c. voltage systems by alock of both converters during the malfunction, i.e., before theresumption of normal operation.

The subsequent transition to normal operation is thus also suitable forthe initial startup of the HVDC transmission line following installationor after a thorough maintenance.

The HVDC transmission line voltage at time t₂₀ rapidly changed by theresumption of normal operation is detected at the latter station, B, attime point t'₂₀, where it leads to the derived release signal QB=1 andsimilarly to the resumption of normal operation.

During a malfunction the switches 60, 61 and 67 of the stations in FIGS.9 and 10 are in a position at which the deactivated controller 41 andrun-up function generator 66 set the control angle 90 degrees inaccordance with the initial setting step of zero. The control angle isrun up at time point t₂₀, in other words, t'₂₀ by activating the run-upfunction generator, whereby position P3 of switch 61 permits temporaryconnection of the supplemental pulse Δα during the system-synchronousstart of the drive unit. If the controllers 41 are activatedsimultaneously with the run-up function generators 66, then the run-upis superimposed by the buildup of oscillations of the controller, whileat time points t₂₁ and t'₂₁ it can be seen from the wave pattern of theautomatic control voltage that now the control angle is being determinedby the automatic control quantities U_(dLA), in other words, α_(BV)which as the measurement value of the fault quantity replaces the modelvalue generated and regulated by the run-up function generator.

(c) Examples with Bypass Operation:

If, in the event of a malfunction in the former station, giveundisturbed operation of the latter station, a bypass operation isspecified due to the already mentioned advantages, then in theundisturbed station upon commencement of the malfunction the bypassthyristors selected for the bypass circuit are fired by the internalmalfunction warning pulse, and the HVDC transmission line connections ofthis station are short-circuited. In the latter station the externalmalfunction warning pulse Fdf initiates a rectifier activity in thistype of operation whereby any desired bypass current is supplied intothe HVDC transmission line. This bypass current is preferrably derivedin accordance with the requirements of the latter station, i.e., basedupon the measured values of the a.c. voltage system available in thatstation.

After the malfunction is over, the bypass circuit is interrupted againin the recuperated station by means of the internal release pulse Ffeand normal operation synchronous with the system resumed, resulting inthe functioning other station in an external release pulse Fff by whichthe bypass current feed is discontinued and normal operation synchronouswith the system resumes. FIGS. 26 and 27 depict advantageous types ofdesigns of said malfunction operation with bypass current.

FIG. 26 assumes a malfunction of the rectifier operation for example, inthe station, A.

The collapsing voltage U_(A) of the system, NA, collapsing at time t₁₀is again recorded in the station, A, by the internal malfunction warningpulse Fde and releases QA=0. Converter 1 is inhibited. The currenti_(dA) thus goes off, and the voltages U_(dA) and U_(dLA), respectivelyare caused to oscillate. Said oscillation is depicted by a broken lineand is dependent on the further events in the station, B.

After a delay time T_(ts), i.e., time T'₁₀, there is a correspondingdrop in voltage U_(dB), in other words, U_(dLB) and in current i_(dB) atthe d.c. connections of station, B. Therefore, by monitoring the currenti_(dB), for example, an external malfunction warning pulse Fdf isgenerated at said location, resulting in Q_(B) =0.

If in the event of said malfunction, bypass operation is foreseen, thenthe functioning station, B, assumes rectifier operation with theexternal malfunction warning pulse, with the control angle beingprovided by a superimposed controller. If, for instance, the bypassoperation is to serve to keep the voltage U_(B) constant or to controlthe reactive load, then a voltage regulator or a reactive powercontroller is provided as a superimposd regulator for the bypassoperation. The output signal of this bypass controller 68B, as shown inFIG. 9, forms value ΔX_(B), which corresponds to the set value i*_(dB)for the current to be taken from the functioning system and is connectedwith the control set as control angle α_(B) in the bypass operation viathe switch 60B. The computing circuit 43B' is separated in thisinstance, e.g., by the switch 61B being switched to the output of therun-up function generator 66B which is inactivated during themalfunction.

In one configuration, at QB=0 at the point in time t'₁₀ first the firingcommands of the converter are inhibited. It then depends on the phaseposition of the system, NB, if and when current i_(dB) extinguishes.Subsequently, rectifier operation is commenced in the station, B, whichresults in a voltage reversal in the HVDC transmission line inaccordance with the polarity of the converter thyristors.

The time t'₁₁ for commencing this rectifier operation (possibly with thepulse Δα being given) is practically freely selectable. The automaticcontrol angle α_(Bv) is switched over to the run-up function generator,which is inactivated during the malfunction, so that the control angleα_(B) is determined solely by the bypass controller which has now beenswitched on.

The bypass operation now taken up by the station, B, has the effect thatan increase in the voltage U_(dA) occurs at the d.c. connections in themalfunctioning the station, A, at time t₁₁, with the storage circuit 63Abeing able to recognize the bypass operation taken up by the functioningstation by means of the simultaneous occurence of signal G711A and theinternal malfunction warning pulse Fde, respectively, and the externalrelease signal Fff derived from the voltage oscillation. As i_(dA) =0 atthis time, it does not matter which converter thyristors of the station,A, connected in series are fired as the bypass circuit. Several parallelbypasses can be closed, for example. In the case at hand a selection ofthose bypass thyristors serving as bypass thyristors among thethyristors of converter 1A required for normal operation ispre-programmed and only a single bypass is foreseen.

The firing commands for this bypass circuit can be formed uponoccurrence of the internal malfunction warning pulse or shortlythereafter; firing is, however, not effected until the voltage U_(dA)due to the voltage impression by the bypass rectifier operation of thestation, B, has changed polarity. Then a current i_(dA) flows throughthe converter 1A and the HVDC transmission line, which, however, isseparated from the malfunctioning network, NA.

The network, NA, resuming operation at time t₂₀ reaches at t₂₁ thepreset limit value at which the internal release pulse is formed. Afterthe time T_(syn) required for the formation of the system-synchronousreference voltage of the drive unit, the malfunction memory QA=1, andthe converter 1A is run up with the control angle α_(A) in a smoothfashion.

In order to avoid a short-circuit current to flow through converter 1B,which in bypass operation initially still operates as a rectifier, andconverter 1A, which has already gone to normal rectifier operation, therun-up of the control angle α_(A) can be delayed as against an initiallyimpressed voltage surge so that the station, B, can make the transitionto inverter operation in a timely manner after recognition of thisvoltage surge.

Furthermore, at time t₂₀ the switch 61A is switched in such a mannerthat the run-up function generator output signal forms the automaticcontrol signal U_(dAv). As this run-up generator is inactivated duringthe malfunction and not released again until the system resumes, thedepicted pattern results for the voltage U_(dAv), with a smooth increaseupon occurrence of the leading release signal QA=1.

At a time t₂₂ allowed by the program circuit the voltage U_(dLA) isagain used as automatic control voltage by switching switch 61A.Therefore during normal operation the control angle α_(A) is practicallygiven by U_(dAv) =U_(dLA), or as the case may be U_(dLB)) and ismodified only slightly by the function of the current regulator. As theclamping circuit inhibits the firing commands of the drive unit STAderived from α_(A) during bypass operation, it is insignificant when inthe interval between t₁₀ and t₂₀ the memory circuit switches fromU_(dAv) =U_(dLA) to the automatic control voltage supplied by the run-upfunction generator. Preferrably the current controller is inactiveduring this time and is not activated until after the return of thesystem via the program circuit, preferrably at the end of the smoothrun-up or upon occurrence at time t₂₁ of the effects of the normaloperation initiated by the other station.

In the station, B, the rectifier operation resumed by the station, A, ina smooth manner effects at t'₂₀ an increase in the voltage U_(dB), or asthe case may be, U_(dLB) and in the current i_(dB), thus triggering therelease signal QB=1 derived here, with which the system is now switchedfrom bypass current control to normal control, i.e., from bypasscontroller 68B to the extinction angle controller 41B in FIG. 9 byswitch 60B. At the same time the run-up function generator is activatedthere with said external release signal of the station, B, which nowsupplies the automatic control angle α_(Bv) for station B instead ofelement 43B'. As a consequence, the control angle α_(B) increases againin a smooth manner to the angle characteristic of normal inverteroperation until at time t'₂₂ the switch 61B is again switched from therun-up function generator to the computing circuit 43B'.

The same principle can be implemented if the station, B, has amalfunction. FIG. 27 assumes this case.

In normal operation the converter 1B in the station, B, is operated asan inverter with a control angle α_(B) near the inverter step limit,which is formed by the extinction angle controller, a controller for thereactive power or another control quantity as the case may be, and iscontrolled with the automatic control angle calculated from theinductive voltage drop. At time t₁₀ the voltage U_(B) of the system, NB,collapses with the effect that via a corresponding signal G711B in themonitoring device of the station, B, the normal firing pulses areinhibited there.

This short circuit also results in the collapse of the d.c. voltageU_(dB) and a rapidly increasing direct current i_(dB) flowing into theshort circuit so that the inverter 1B becomes unstable. The capacitanceof the HVDC transmission line at the station, B, connection are thusdischarged into the short circuit, resulting in a reversal of thevoltage U_(dLB) and finally in an extinction of the direct currenti_(dB). Meanwhile the limit value warning device has also effectedcondition QB=0 via the internal malfunction warning pulse, by which thenormal commutation of the converter 1B is inhibited and the firing ofthe bypass thryristors initiated.

According to the line's distributed time delay, the d.c. current i_(dA)increases in the station, A, and the d.c. voltages U_(dA) and U_(dLA)drop accordingly. Via the external malfunction warning pulse of thestation, A, this results in condition QA=0 at which the operationalcurrent set value i*_(dA) is switched to a lower set value which issupplied by a superimposed bypass controller, e.g., the reactive powercontroller 68A or a voltage regulator for voltage U_(A) (switch 60A inFIG. 10). The converter 1A therefore feeds only the reactive currentinto the HVDC transmission line which is required for as continuous anoperation as possible of network, NA. Furthermore, the signal QA=0triggers the switching of the automatic control voltage U_(dAv) from themeasuring value output for U_(dLA) to the output of the run-up functiongenerator by switch 61A switching at a preset later time t₁₂, forexample.

As the station, A, continues to feed current into the HVDC transmissionline, the end of said transmission line discharged through the systemshort circuit is recharged in the station B. Thus the voltage U_(dA)acquires positive values again i.e., at time t₁₂, so that upon attaininga preset positive limit value, certain thyristors fire in the station,B, which are selected for the formation of the bypass circuit and atwhich corresponding firing voltages arise at time t₁₂. Thus the bypasscircuit is now closed and the bypass operation initiated during whichthe HVDC transmission line is operated as reactive impedance for thesystem, NA.

In the case depicted by FIG. 27 the inhibiting of the normal firingcommands supplied by the drive unit took place at the time t₁₀ in themalfunctioning station, B, with QB=0, whereby the current i_(dB)initially continues to flow through the thyristors of converter 1Binvolved in the shutdown of converter 1B. Only after extinction of thesethyristors the HVDC transmission line has been recharged with a currentwhich at time t₁₂ leads to a positive response value of U_(dB), whilethe station, A, has started the bypass operation, is the voltage appliedto the thyristors selected for bypass operation, which voltage resultsin the firing of the bypass thyristors and thus in a recurring currenti_(dB). In this event the bypass thyristors can be selected independentof the thyristors involved in the inverter shutdown. However, as in thestation, B, the thyristors involved in the shutdown can be selected asbypass thyristors independent of operation, a frequently undesirablecomplete extinction of the current i_(dB) must not be waited for.

Particularly in the case depicted in FIG. 27 substantial negative valuesof U_(dB) can result in the time interval between t₁₀ and t₁₂, which theusers try to prevent in many cases when operating a HVDC transmissionline.

This can be achieved if the control angle α_(A) of the station, A, isnot immediately reduced to a value near zero corresponding to the bypassoperation when the derived malfunction signal QA=0, but rather to acontrol angle which is initially shifted in the direction of a rectifierwide-open setting in order to recharge the HVDC transmission line asquickly as possible. This value can be given by the storage circuit 63Avia the position P3 of switch 61A according to FIG. 10. If a networkmonitor 43A' is available, however, as explained in FIGS. 13 and 14, itis then possible to keep the current i_(dB) almost constant, bycontrolling the substitute actual value i_(dB) of the current regulator41A' calculated by the network monitor to a set value supplied by thevoltage regulator 68A serving to maintain a constant voltage U_(A). Iffor normal operation, for example, a superimposed active power regulator51 is provided, then through QA=0 this superimposed regulator isswitched to a bypass controller, such as, voltage regulator 68A, whilein the observer station the inverter shutdown of malfunctioning station,B, is simulated by the closing of switch 77.

In order to convert from the bypass operation back to thesystem-synchronous normal operation upon the return of the network, NB,at time t₂₀ as shown in FIG. 7 and upon occurrence of the leadingrelease signal QB=1, those thyristors are fired in the already describedmanner which have been pre-programmed with a pre-programmed bypassthyristor combination in order to initiate the system-synchronousoperation while impressing a voltage surge, or those thyristors arefired which have been selected by the selector switch dependent on theoperation, in accordance with the bypass thyristors fired dependent ofoperation.

FIG. 27 depicts the voltage U_(dB) occurring after the time T_(syn)required for synchronization of the reference voltage generator, whichvoltage is a function of the run-up of α_(B). At the time t'₂₀ then therelease signal QA=1 occurs, with a corresponding increase of the angleα_(A) to the rectifier wide-open setting provided for normal operationand a corresponding pattern of voltage U_(dA), with the bypass controlnow discontinued in the functioning station, A, and normal control beingswitched on via the current controller. At times t₂₂ and t'₂₂ switchingfrom the automatic control value preset as a controlled run-up functionto the measured fault quantity as a automatic control quantity takesplace.

In this design the particular advantage is that during normal operationthe two stations function independent of one another, i.e., that noinformation to be transmitted via remote control lines is required fromthe respective other station for the control of the two converters.Therefore both converters can be quickly controlled by correspondingautomatic control without forcing a slow control pattern due to thedelay time of remote control signal transmission. Even in the event ofmalfunction the transmission of corresponding malfunction signals is noteffected via remote control lines, but via the HVDC transmission lineitself so that in the event of a malfunction in the one station thenecessary information on the malfunction is available in the otherstation as well within the shortest time possible. The resumption of thefault-free normal operation in the one station is communicated in thesame way within the shortest possible time so that very short on-controltimes result for the resumption of normal operation. Furthermore, it ispossible by said rapid control to utilize he HVDC transmission lineitself to control or regulate the electrical quantities of therespective systems, e.g., for reactive current control; or during bypassoperation to maintain a fault-free system constant, or to dampen otherprocesses, e.g., balancing processes in the systems.

As will be evident from the foregoing description, certain aspects ofthe invention are not limited to the particular details of the examplesillustrated, and it is therefore contemplated that other modificationsor applications will occur to those skilled in the art. It isaccordingly intended that the claims shall cover all such modificationsand applications as do not depart from the true spirit and script of theinvention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A method for operating a HVDC transmission linesystem connected between two alternating current systems, a firstnetwork and a second network, during emergency operation resulting froma malfunction having a first station, connected to the first networkwhich during normal operation operates as a system-synchronous firstconverter rectifying the incoming alternating current and impressing anormal d.c. current through the HVDC transmission line, and a secondstation, connected to the second network, determining the normal voltageof the HVDC transmission line, further forming a leading release signalsubsequent to the end of the malfunction; wherein the second stationduring normal operation operates as a system-synchronous secondconverter inverting the incoming HVDC and impressing a normal a.c.current into the second network; said method being adapted to form ashort circuit as a bypass circuit around a malfunction in eitherstation, comprising the steps of:forming a leading fault indicationsignal at the onset of the malfunction in the malfunctioning station;forming the bypass circuit from bypass thyristors which are designed towithstand as a normal operating condition a load current correspondingto the level of reactive current encountered during the malfunctionbypass operation, by firing said bypass thyristors by means of saidleading fault indication signal; forming subsequently in the functioningstation a derived fault indication signal; stimulating a rectifieroperation of a.c. current into bypass d.c. current in the functioningstation by said derived fault indication signal; impressing said bypassd.c. current through the HVDC transmission line and the bypass circuit;interrupting the bypass circuit causing said bypass thyristors to becomenon-conducting by the leading release signal subsequent to the end ofthe malfunction; initiating a transition of the previouslymalfunctioning station to system-synchronous normal operation by theleading release signal; forming a derived release signal in thepreviously functioning station; terminating said rectifier operation bythe derived release signal; and said derived release signal furtherinitiating a transition of the previously functioning station from saidrectifier operation to system-synchronous normal operation.
 2. Themethod according to claim 1 further including the steps of:presettingsaid bypass d.c. current in accordance with the requirements of the a.c.system, which is connected to the functioning station, particularly inaccordance with the output signal of a bypass controller for said a.c.system which is connected to a functioning station voltage signal orreactive power signal whereby said bypass controller is located in thestation supplying said bypass d.c. current.
 3. A method according toclaim 1, further including the steps of shifting a control angle duringmalfunction operation in the converter in the functioning station torectifier operation by means of said derived fault indication signal;and impressing a voltage, during malfunction operation by saidfunctioning station, across the HVDC transmission line which is positivein the direction of the current flow direction of said bypass thyristorsin the malfunctioning station and of sufficient level to fire saidbypass thyristors.
 4. A method in accordance with claim 3, furtherincluding the steps of:adding to said control angle during malfunction apulse initiating a temporary voltage pulse on the HVDC transmission linefor assumption of malfunction operation, particularly adding said pulseto the control angle of the second station for the rapid recharging ofthe HVDC transmission line in the event of a malfunction in the firststation.
 5. A method in accordance with claim 3, further including thesteps of:discharging the HVDC transmission line via series connectedthyristors in the second station in the event of a malfunction in thesecond station, with subsequent firing of said bypass thyristors in thesecond station after impressing by the first station of a positivevoltage sufficient to fire the bypass thyristors on the HVDCtransmission line.
 6. A method according to claim 3, further includingthe steps of:setting of a leading fault indication signal in the secondstation by the occurring of a malfunction in the second station; firingimmediately, thereafter by means of said leading fault indicationsignal, said bypass thyristors; and setting a derived fault indicationsignal in the first station; and presetting a control angle from saidderived fault indication signal; whereby the HVDC transmission line isrecharged without the extinction of the HVDC transmission line current.7. A method according to claim 6, further comprising the stepsof:forming said control angle in such a manner as to provide an input toa system simulation model located in the first station; forming fromsaid control angle input to said system simulation model, a modelquantity, which is proportional to the d.c. current of the secondstation; and controlling said model quantity to a value of maximumconstancy.
 8. A method according to claim 1, in which said bypassthyristors that are being fired are a selected combination of thethyristors used in the malfunctioning converter during normal operation.9. A method according to claim 8, further comprising the stepsof:presetting a start value of a control angle in the previouslymalfunctioning station for resumption of system-synchronous normaloperation following the end of the malfunction; forming from said startvalue of said control angle and from a system-voltage-synchronizedreference voltage the normal system-synchronous operation firing commandsequence; and selecting for a start of said normal system-synchronousoperation firing command sequence, a firing command in said sequencecorresponding to the decommutation of the current from one of saidbypass thyristors in said firing command sequence.
 10. A methodaccording to claim 9, in which said combination of bypass thyristors andselection of said start firing commands are preprogrammed.
 11. A methodaccording to claim 9, further comprising the steps of:determining, as afunction of which thyristors are conducting current at the occurrence ofsaid leading fault indication signal, said combination of bypassthyristors; and selecting, as a function of the combination of bypassthyristors determined as a function of the occurrence of said leadingfault indication signal, the firing commands to start normal operation.12. A method according to claim 11determining, from the firing commandlast given during normal operation from the respective sequence of thesystem-synchronous firing commands, said bypass thyristor combination;storing said determined bypass thyristor combination for subsequentfiring of thyristors; inhibiting the firing commands by the occurrenceof the leading fault indication signal; and firing said stored bypassthyristor combination thyristors.
 13. A method according to claim 12further comprising the steps of:setting a memory unit upon firing of thebypass thyristors; reading out, subsequently, at the occurrence of saidleading release signal, information stored in said memory unit releasingthe firing command selected thereby for start-up of normal operation.14. A method according to claim 9 further comprising the stepsof:forming, by monitoring the a.c. voltage system, said leading releasesignal; suppressing, initially, said leading release signal following amalfunction of the system until the synchronizing of a reference voltagegenerator within a maximum permissible synchronization error of 30degrees.
 15. A method according to claim 1, further comprising the stepof:resuming of normal operation in the non-malfunctioning stationoccurring as a result of impressing on the HVDC transmission line avoltage pulse corresponding to temporary rectifier operation.
 16. Amethod according to claim 1, further comprising the step of:resumingnormal operation of a converter after either a leading release signal ora derived release signal including the running up of the control anglefrom a value approximately the value of the converter output d.c.voltage of zero, to a final value approximately corresponding to normaloperation, having the start of the run-up function capable of beingpreceded by a temporary control angle shifting to a rectifier operationor a respective superimposing.
 17. A method according to claim 16, inwhich said run-up time is approximately two periods of the fundamentala.c. system voltage.
 18. A method according to claim 16, in which therunning up of the control angles of the first and the second convertersis pre-programmed and mutually compatible.
 19. A method according toclaim 16, further comprising the step of:presetting a control angle, inat least one of the stations by an automatic control apparatus, afterthe running up of said control angle, by connection to the output signalof a control quantity controller and to a model value for the faultindication quantity generated by the functioning station in normaloperation.
 20. A method according to claim 19, in which a voltage, isconnected in the first station as a model value for the case of a HVDCremote transmission line at the end of a converter reactance coilfollowing the first converter on the d.c. voltage side.
 21. A methodaccording to claim 19, for the case of a HVDC transmission line shortcoupling further comprising the step of connecting in the first stationas a model value presetting a model quantity formed by a computedinductive d.c. voltage drop of the second converter.
 22. A methodaccording to claim 19, further comprising the step of:forming a modelquantity in the first station by a model simulation circuit driven byoperational data of the first station.
 23. A method according to claim19, further comprising the steps of:determining, from the inductive d.c.voltage drop of the second converter, a model quantity, whereby saidinductive d.c. voltage drop is computed from a given set extinctionangle preset from the set value of the control quantity controller andmeasured values of the d.c. current and the system d.c. voltage of thesecond station, added to the output signal of a control quantitycontroller, such as a controller for the voltage or the controller forthe transmitted reactive power.
 24. A method according to claim 16,further comprising the steps of:presetting of said control angle of atleast one of the stations by an automatic control apparatus, saidapparatus which is switchably connected to a bypass controller outputsignal determining the bypass current, such as a voltage regulator forthe a.c. voltage of the malfunctioning system or of a reactive outputcontrol during malfunction condition operation; and during normaloperation to the output signal of a control quantity controller which isadditionally connected to a run-up function generator during run-up andto the model value of the fault indication quantity during normaloperation, and preferrably up to the start of the run-up function.
 25. Amethod in accordance with claim 16, in which said control angle of saidfirst station during the transition from emergency operation to normaloperation initially having a larger and later decreasing run-upincrement to normal rectifier operation, and said control angle of saidsecond station having a run-up to normal inverter operation in theopposite increment sequence from that of said first station.
 26. Amethod in accordance with claim 16, further comprising the stepsof:staggering the start up times of component current converters inconverters consisting of several component current converters duringnormal operation, whereby the component current converter starting firstis rapidly run up to a control angle preset by the operation and thentaken back to the extent that the run-up of the other component currentconverters is complete.
 27. A method in accordance with claim 1, inwhich said leading fault indication signal and the leading releasesignal are generated from operating data of the malfunctioningconverter, such as from measured values of the a.c. system voltage, andsaid derived fault indication signal and said derived release signal aregenerated from measured values of the d.c. connections of thefunctioning station as soon as effects of the interrupted or resumednormal operation are detected therein.
 28. An apparatus for HVDCtransmission, including:a first station connecting to a first a.c.system to draw electrical power therefrom; a first converter being partof said first station and being connected to said first a.c. system; andoperating as a system-synchronous rectifier during normal operation; aHVDC transmission line being connected at one end to said firstconverter; a second converter being connected to the other end of saidHVDC transmission line and operating as a system-synchronous inverterduring normal operation; a second station having said second converter apart thereof and having connections thereto; a second a.c. voltagesystem being connected to said second station; a first reference voltagegenerator located in and connected to the first station for generating asystem-synchronous reference voltage with respect to the first a.c.system; a second reference voltage generator located in and connected tothe second station for generating a system-synchronous reference voltagewith respect to the second a.c. system; a first controller forming afirst control angle, connected to the first station; a second controllerforming a second control angle, connected to the second station B; afirst drive unit connecting to said first controller for forming a firstset of system-synchronous firing commands; a second drive unitconnecting to said second controller for forming a second set ofsystem-synchronous firing commands; a first monitoring means monitoringthe values of the electrical quantities of the first station and formingtherefrom a first fault indication signal when a malfunction isindicated and a first release signal when return to normal operation isindicated; a second monitoring means monitoring the values of theelectrical quantities of the second station and forming therefrom asecond fault indication signal when a malfunction is indicated and asecond release signal when return to normal operation is indicated; afirst clamping circuit connected to a first drive unit and said firstmonitoring means inhibiting the transmission of said first firingcommands whenever said first monitoring means forms said first faultindication signal, and transmitting said first fixing commands wheneversaid first monitoring means forms said first release signal; and asecond clamping circuit connected to a second drive unit and said secondmonitoring means inhibiting the transmission of said second firingcommands whenever said second monitoring means forms said second faultindication signal, and transmitting said second firing commands wheneversaid second monitoring means forms said second release signal,comprising: a first generating means as a part of the first monitoringmeans for generating a first leading fault indication signal whenmonitoring of the first a.c. system electrical quantities indicate theoccurrence of a malfunction relative to the first station; a secondgenerating means as a part of the second monitoring means for generatinga second leading fault indicator signal when monitoring of the seconda.c. system electrical quantities indicate the occurrence of amalfunction relative to the second station; a third generating means asa part of the first monitoring means for generating a first leadingrelease signal when monitoring of the first a.c. system electricalquantities indicates discontinuance of a malfunction relative to thefirst station; a fourth generating means as a part of the secondmonitoring means for generating a second leading release signal whenmonitoring the second a.c. system electrical quantities indicates thediscontinuance of a malfunction relative to the second station; a fifthgenerating means as a part of the first monitoring means for generatinga first derived fault indication signal when monitoring the d.c. voltageelectrical quantities indicates the occurrence of a malfunction in alocation other than the first station; a sixth generating means as apart of the second monitoring means for generating a second derivedfault indication signal when monitoring the d.c. voltage electricalquantities indicates the occurrence of a malfunction in a location otherthan the second station; a seventh generating means as a part of thefirst monitoring means for generating a first derived release signalwhen monitoring the d.c. voltage electrical quantities indicates adiscontinuance of malfunction outside of the first station; an eighthgenerating means as a part of the second monitoring means for generatinga second derived release signal when monitoring the d.c. voltageelectrical quantities indicates a discontinuance of malfunction outsideof the second station; a first memory as part of the first clampingcircuit of the first station storing a combination of converterthyristors selected as bypass thyristors in the first station; a secondmemory as part of the second clamping circuit of the second stationstoring a combination of converter thyristors selected as bypassthyristors in the second station; a first and second switching meansproximately located and connected to said first and second memoryrespectively, and further connected respectively to converter thyristorsin the first and second converters, inhibiting the system synchronousfiring commands to said converter thyristors of the respective stationwhen a leading fault indication signal occurs therein, and generatingbypass firing commands to selected bypass thyristors of said respectivestation; a first and second selector circuit connected to andproximately located to the first and second drive unit respectively,having a third and fourth memory respectively, contained therein,whereby after the firing of bypass thyristors in the respective station,a combination of start thyristors is stored for resumption of normaloperation of the respective station; a first and second logic circuitconnected to said first and second selector circuit respectively,eliminating the inhibiting of system-synchronous firing commands aftersaid respective leading release signal has occurred, and subsequently afiring command for one of the thyristors of said respective startingthyristor combination occurs; and a first and second pre-programmedcircuit, connected to the first and second control angle controllersrespectively, interacting with the respective control angle in such amanner that after the start of the respective derived fault indicationsignal, the converter in the functional station is operated as arectifier in bypass operation, until the occurrence of the respectivederived release signal at which time bypass operation is terminated andnormal operation resumed.
 29. An apparatus according to claim 28,further comprising:a third and fourth switching means, connecting to thefirst and second drive unit respectively, whereby during bypassoperation of one of the stations said switching means in the other,functioning station provides an electrical connection from the driveunit and a bypass controller of the other station.
 30. An apparatusaccording to claim 29, whereby said bypass controller is a voltageregulator.
 31. An apparatus according to claim 29, whereby said bypasscontroller is a reactive power controller.
 32. An apparatus according toclaim 28, further comprising:a third and fourth switching means,connected to the first and second drive unit respectively, connecting afirst and second run-up function generator respectively to therespective drive unit when said respective release signal occurs forminga respective firing angle.
 33. An apparatus according to claim 32,further comprising:a first and second device for determining a faultindication quantity respectively in the first and second station,whereby after completion of the respective run-up function, said thirdand fourth switching means switch from connection to respective run-upfunction generator, to connection to said device for determining a faultindication respectively.
 34. An apparatus according to claim 33,whereby:said first or second fault indication quantity determiningdevice in the first station is connected to monitor the d.c. voltagefrom the first station; and the inductive d.c. voltage drop of therespective converter in the second station.
 35. An apparatus accordingto claim 33, further comprising:a first and second automatic controldevice connected in series with an input to said first and second driveunit respectively, connecting to the respective fault indicationquantity and the output signal of the respective control quantitycontroller as inputs upon the occurrence of a release signal.
 36. Anapparatus according to claim 32, whereby said clamping circuits,selector circuits, and run-up function generators are used by theirrespective component current converter of their respective converters toprovide resumption of normal operation in a time-staggered fashion andby different run-up functions in the converters.
 37. An apparatusaccording to claim 28, further comprising:a first and second memoriescontrol means connected to said first and second memories, respectively,and to said third and fourth memories, respectively, setting said firstand second memories at the occurrence of an output signal from therespective drive unit to a respective bypass thyristor combinationdependent upon the momentary control status of the respective converterthyristors; and setting said third and fourth memories to a respectivestart thyristor combination dependent on the bypass thyristors.
 38. Anapparatus according to claim 28, whereby:said first and secondmonitoring device in order to provide the respective fault indicationrelease signals; and said control quantity controllers in order toprovide the respective control angle in the respective station; havingan electrical quantity inputs of actual and set values provided in therespective stations without the use of remote control signals.
 39. Anapparatus according to claim 28, whereby said memories of said clampingcircuits, and said memories of said selector circuits being,read-only-memories, preferably contained in said pre-programmingcircuit.